runtime_control.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<sect1 id="runtime-control">
  <title>Running a compiled program</title>

  <indexterm><primary>runtime control of Haskell programs</primary></indexterm>
  <indexterm><primary>running, compiled program</primary></indexterm>
  <indexterm><primary>RTS options</primary></indexterm>

  <para>To make an executable program, the GHC system compiles your
  code and then links it with a non-trivial runtime system (RTS),
  which handles storage management, thread scheduling, profiling, and
  so on.</para>

  <para>
    The RTS has a lot of options to control its behaviour.  For
    example, you can change the context-switch interval, the default
    size of the heap, and enable heap profiling.  These options can be
    passed to the runtime system in a variety of different ways; the
    next section (<xref linkend="setting-rts-options" />) describes
    the various methods, and the following sections describe the RTS
    options themselves.
  </para>

  <sect2 id="setting-rts-options">
    <title>Setting RTS options</title>
    <indexterm><primary>RTS options, setting</primary></indexterm>

    <para>
      There are four ways to set RTS options:

      <itemizedlist>
        <listitem>
          <para>
          on the command line between <literal>+RTS ... -RTS</literal>, when running the program
           (<xref linkend="rts-opts-cmdline" />)
          </para>
        </listitem>
        <listitem>
          <para>at compile-time, using <option>--with-rtsopts</option>
            (<xref linkend="rts-opts-compile-time" />)
          </para>
        </listitem>
        <listitem>
          <para>with the environment variable <envar>GHCRTS</envar>
          (<xref linkend="rts-options-environment" />)
          </para>
        </listitem>
        <listitem>
          <para>by overriding &ldquo;hooks&rdquo; in the runtime system
            (<xref linkend="rts-hooks" />)
          </para>
        </listitem>
      </itemizedlist>
    </para>

    <sect3 id="rts-opts-cmdline">
      <title>Setting RTS options on the command line</title>

      <para>
        If you set the <literal>-rtsopts</literal> flag appropriately
        when linking (see <xref linkend="options-linker" />), you can
        give RTS options on the command line when running your
        program.
      </para>

      <para>
        When your Haskell program starts up, the RTS extracts
        command-line arguments bracketed between
        <option>+RTS</option><indexterm><primary><option>+RTS</option></primary></indexterm>
        and
        <option>-RTS</option><indexterm><primary><option>-RTS</option></primary></indexterm>
        as its own.  For example:
      </para>

<screen>
$ ghc prog.hs -rtsopts
[1 of 1] Compiling Main             ( prog.hs, prog.o )
Linking prog ...
$ ./prog -f +RTS -H32m -S -RTS -h foo bar
</screen>

        <para>
          The RTS will
          snaffle <option>-H32m</option> <option>-S</option> for itself,
          and the remaining arguments <literal>-f -h foo bar</literal>
          will be available to your program if/when it calls
          <function>System.Environment.getArgs</function>.
        </para>

        <para>
          No <option>-RTS</option> option is required if the
          runtime-system options extend to the end of the command line, as in
          this example:
        </para>

<screen>
% hls -ltr /usr/etc +RTS -A5m
</screen>

        <para>
          If you absolutely positively want all the rest of the options
          in a command line to go to the program (and not the RTS), use a
          <option>--RTS</option><indexterm><primary><option>--RTS</option></primary></indexterm>.
        </para>

        <para>
          As always, for RTS options that take
          <replaceable>size</replaceable>s: If the last character of
          <replaceable>size</replaceable> is a K or k, multiply by 1000; if an
          M or m, by 1,000,000; if a G or G, by 1,000,000,000.  (And any
          wraparound in the counters is <emphasis>your</emphasis>
          fault!)
        </para>

        <para>
          Giving a <literal>+RTS -?</literal>
          <indexterm><primary><option>-?</option></primary><secondary>RTS option</secondary></indexterm> option
          will print out the RTS options actually available in your program
          (which vary, depending on how you compiled).</para>

        <para>
          NOTE: since GHC is itself compiled by GHC, you can change RTS
          options in the compiler using the normal
          <literal>+RTS ... -RTS</literal>
          combination.  eg. to set the maximum heap
          size for a compilation to 128M, you would add
          <literal>+RTS -M128m -RTS</literal>
          to the command line.
        </para>
      </sect3>

      <sect3 id="rts-opts-compile-time">
        <title>Setting RTS options at compile time</title>

        <para>
          GHC lets you change the default RTS options for a program at
          compile time, using the <literal>-with-rtsopts</literal>
          flag (<xref linkend="options-linker" />).  A common use for this is
          to give your program a default heap and/or stack size that is
          greater than the default.  For example, to set <literal>-H128m
            -K64m</literal>, link
          with <literal>-with-rtsopts="-H128m -K64m"</literal>.
        </para>
      </sect3>

      <sect3 id="rts-options-environment">
        <title>Setting RTS options with the <envar>GHCRTS</envar>
          environment variable</title>

        <indexterm><primary>RTS options</primary><secondary>from the environment</secondary></indexterm>
        <indexterm><primary>environment variable</primary><secondary>for
            setting RTS options</secondary></indexterm>

        <para>
          If the <literal>-rtsopts</literal> flag is set to
          something other than <literal>none</literal> when linking,
          RTS options are also taken from the environment variable
          <envar>GHCRTS</envar><indexterm><primary><envar>GHCRTS</envar></primary>
          </indexterm>.  For example, to set the maximum heap size
          to 2G for all GHC-compiled programs (using an
          <literal>sh</literal>-like shell):
        </para>

<screen>
   GHCRTS='-M2G'
   export GHCRTS
</screen>

        <para>
          RTS options taken from the <envar>GHCRTS</envar> environment
          variable can be overridden by options given on the command
          line.
        </para>

        <para>
          Tip: setting something like <literal>GHCRTS=-M2G</literal>
          in your environment is a handy way to avoid Haskell programs
          growing beyond the real memory in your machine, which is
          easy to do by accident and can cause the machine to slow to
          a crawl until the OS decides to kill the process (and you
          hope it kills the right one).
        </para>
      </sect3>

  <sect3 id="rts-hooks">
    <title>&ldquo;Hooks&rdquo; to change RTS behaviour</title>

    <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
    <indexterm><primary>RTS hooks</primary></indexterm>
    <indexterm><primary>RTS behaviour, changing</primary></indexterm>

    <para>GHC lets you exercise rudimentary control over certain RTS
    settings for any given program, by compiling in a
    &ldquo;hook&rdquo; that is called by the run-time system.  The RTS
    contains stub definitions for these hooks, but by writing your
    own version and linking it on the GHC command line, you can
    override the defaults.</para>

    <para>Owing to the vagaries of DLL linking, these hooks don't work
    under Windows when the program is built dynamically.</para>

    <para>You can change the messages printed when the runtime
    system &ldquo;blows up,&rdquo; e.g., on stack overflow.  The hooks
    for these are as follows:</para>

    <variablelist>

      <varlistentry>
	<term>
          <function>void OutOfHeapHook (unsigned long, unsigned long)</function>
          <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
        </term>
	<listitem>
	  <para>The heap-overflow message.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <function>void StackOverflowHook (long int)</function>
          <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
        </term>
	<listitem>
	  <para>The stack-overflow message.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <function>void MallocFailHook (long int)</function>
          <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
        </term>
	<listitem>
	  <para>The message printed if <function>malloc</function>
	  fails.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </sect3>

    </sect2>

  <sect2 id="rts-options-misc">
    <title>Miscellaneous RTS options</title>

    <variablelist>
     <varlistentry>
       <term><option>-V<replaceable>secs</replaceable></option>
       <indexterm><primary><option>-V</option></primary><secondary>RTS
       option</secondary></indexterm></term>
       <listitem>
         <para>Sets the interval that the RTS clock ticks at.  The
         runtime uses a single timer signal to count ticks; this timer
         signal is used to control the context switch timer (<xref
         linkend="using-concurrent" />) and the heap profiling
         timer <xref linkend="rts-options-heap-prof" />.  Also, the
         time profiler uses the RTS timer signal directly to record
         time profiling samples.</para>

         <para>Normally, setting the <option>-V</option> option
         directly is not necessary: the resolution of the RTS timer is
         adjusted automatically if a short interval is requested with
         the <option>-C</option> or <option>-i</option> options.
         However, setting <option>-V</option> is required in order to
         increase the resolution of the time profiler.</para>

         <para>Using a value of zero disables the RTS clock
         completely, and has the effect of disabling timers that
         depend on it: the context switch timer and the heap profiling
         timer.  Context switches will still happen, but
         deterministically and at a rate much faster than normal.
         Disabling the interval timer is useful for debugging, because
         it eliminates a source of non-determinism at runtime.</para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>--install-signal-handlers=<replaceable>yes|no</replaceable></option>
       <indexterm><primary><option>--install-signal-handlers</option></primary><secondary>RTS
       option</secondary></indexterm></term>
       <listitem>
         <para>If yes (the default), the RTS installs signal handlers to catch
         things like ctrl-C. This option is primarily useful for when
         you are using the Haskell code as a DLL, and want to set your
         own signal handlers.</para>

         <para>Note that even
           with <option>--install-signal-handlers=no</option>, the RTS
           interval timer signal is still enabled.  The timer signal
           is either SIGVTALRM or SIGALRM, depending on the RTS
           configuration and OS capabilities.  To disable the timer
           signal, use the <literal>-V0</literal> RTS option (see
           above).
         </para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>-xm<replaceable>address</replaceable></option>
       <indexterm><primary><option>-xm</option></primary><secondary>RTS
       option</secondary></indexterm></term>
       <listitem>
         <para>
           WARNING: this option is for working around memory
           allocation problems only.  Do not use unless GHCi fails
           with a message like &ldquo;<literal>failed to mmap() memory below 2Gb</literal>&rdquo;.  If you need to use this option to get GHCi working
           on your machine, please file a bug.
         </para>

         <para>
           On 64-bit machines, the RTS needs to allocate memory in the
           low 2Gb of the address space.  Support for this across
           different operating systems is patchy, and sometimes fails.
           This option is there to give the RTS a hint about where it
           should be able to allocate memory in the low 2Gb of the
           address space.  For example, <literal>+RTS -xm20000000
           -RTS</literal> would hint that the RTS should allocate
           starting at the 0.5Gb mark.  The default is to use the OS's
           built-in support for allocating memory in the low 2Gb if
           available (e.g. <literal>mmap</literal>
           with <literal>MAP_32BIT</literal> on Linux), or
           otherwise <literal>-xm40000000</literal>.
         </para>
       </listitem>
     </varlistentry>
    </variablelist>
  </sect2>

  <sect2 id="rts-options-gc">
    <title>RTS options to control the garbage collector</title>

    <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
    <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>

    <para>There are several options to give you precise control over
    garbage collection.  Hopefully, you won't need any of these in
    normal operation, but there are several things that can be tweaked
    for maximum performance.</para>

    <variablelist>

      <varlistentry>
	<term>
          <option>-A</option><replaceable>size</replaceable>
          <indexterm><primary><option>-A</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>allocation area, size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 512k&rsqb; Set the allocation area size
          used by the garbage collector.  The allocation area
          (actually generation 0 step 0) is fixed and is never resized
          (unless you use <option>-H</option>, below).</para>

	  <para>Increasing the allocation area size may or may not
          give better performance (a bigger allocation area means
          worse cache behaviour but fewer garbage collections and less
          promotion).</para>

	  <para>With only 1 generation (<option>-G1</option>) the
          <option>-A</option> option specifies the minimum allocation
          area, since the actual size of the allocation area will be
          resized according to the amount of data in the heap (see
          <option>-F</option>, below).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-c</option>
          <indexterm><primary><option>-c</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>garbage collection</primary><secondary>compacting</secondary></indexterm>
          <indexterm><primary>compacting garbage collection</primary></indexterm>
        </term>
	<listitem>
	  <para>Use a compacting algorithm for collecting the oldest
	  generation.  By default, the oldest generation is collected
	  using a copying algorithm; this option causes it to be
	  compacted in-place instead.  The compaction algorithm is
	  slower than the copying algorithm, but the savings in memory
	  use can be considerable.</para>

	  <para>For a given heap size (using the <option>-H</option>
	  option), compaction can in fact reduce the GC cost by
	  allowing fewer GCs to be performed.  This is more likely
	  when the ratio of live data to heap size is high, say
	  &gt;30&percnt;.</para>

	  <para>NOTE: compaction doesn't currently work when a single
	  generation is requested using the <option>-G1</option>
	  option.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-c</option><replaceable>n</replaceable></term>

	<listitem>
	  <para>&lsqb;Default: 30&rsqb; Automatically enable
	  compacting collection when the live data exceeds
	  <replaceable>n</replaceable>&percnt; of the maximum heap size
	  (see the <option>-M</option> option).  Note that the maximum
	  heap size is unlimited by default, so this option has no
	  effect unless the maximum heap size is set with
	  <option>-M</option><replaceable>size</replaceable>. </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-F</option><replaceable>factor</replaceable>
          <indexterm><primary><option>-F</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, factor</primary></indexterm>
        </term>
	<listitem>

	  <para>&lsqb;Default: 2&rsqb; This option controls the amount
          of memory reserved for the older generations (and in the
          case of a two space collector the size of the allocation
          area) as a factor of the amount of live data.  For example,
          if there was 2M of live data in the oldest generation when
          we last collected it, then by default we'll wait until it
          grows to 4M before collecting it again.</para>

	  <para>The default seems to work well here.  If you have
          plenty of memory, it is usually better to use
          <option>-H</option><replaceable>size</replaceable> than to
          increase
          <option>-F</option><replaceable>factor</replaceable>.</para>

	  <para>The <option>-F</option> setting will be automatically
          reduced by the garbage collector when the maximum heap size
          (the <option>-M</option><replaceable>size</replaceable>
          setting) is approaching.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-G</option><replaceable>generations</replaceable>
          <indexterm><primary><option>-G</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>generations, number of</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 2&rsqb; Set the number of generations
          used by the garbage collector.  The default of 2 seems to be
          good, but the garbage collector can support any number of
          generations.  Anything larger than about 4 is probably not a
          good idea unless your program runs for a
          <emphasis>long</emphasis> time, because the oldest
          generation will hardly ever get collected.</para>

	  <para>Specifying 1 generation with <option>+RTS -G1</option>
          gives you a simple 2-space collector, as you would expect.
          In a 2-space collector, the <option>-A</option> option (see
          above) specifies the <emphasis>minimum</emphasis> allocation
          area size, since the allocation area will grow with the
          amount of live data in the heap.  In a multi-generational
          collector the allocation area is a fixed size (unless you
          use the <option>-H</option> option, see below).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-qg<optional><replaceable>gen</replaceable></optional></option>
          <indexterm><primary><option>-qg</option><secondary>RTS
          option</secondary></primary></indexterm>
        </term>
        <listitem>
          <para>&lsqb;New in GHC 6.12.1&rsqb; &lsqb;Default: 0&rsqb;
            Use parallel GC in
            generation <replaceable>gen</replaceable> and higher.
            Omitting <replaceable>gen</replaceable> turns off the
            parallel GC completely, reverting to sequential GC.</para>

          <para>The default parallel GC settings are usually suitable
            for parallel programs (i.e. those
            using <literal>par</literal>, Strategies, or with multiple
            threads).  However, it is sometimes beneficial to enable
            the parallel GC for a single-threaded sequential program
            too, especially if the program has a large amount of heap
            data and GC is a significant fraction of runtime.  To use
            the parallel GC in a sequential program, enable the
            parallel runtime with a suitable <literal>-N</literal>
            option, and additionally it might be beneficial to
            restrict parallel GC to the old generation
            with <literal>-qg1</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-qb<optional><replaceable>gen</replaceable></optional></option>
          <indexterm><primary><option>-qb</option><secondary>RTS
          option</secondary></primary></indexterm>
        </term>
        <listitem>
          <para>
            &lsqb;New in GHC 6.12.1&rsqb; &lsqb;Default: 1&rsqb; Use
            load-balancing in the parallel GC in
            generation <replaceable>gen</replaceable> and higher.
            Omitting <replaceable>gen</replaceable> disables
            load-balancing entirely.</para>

          <para>
            Load-balancing shares out the work of GC between the
            available cores.  This is a good idea when the heap is
            large and we need to parallelise the GC work, however it
            is also pessimal for the short young-generation
            collections in a parallel program, because it can harm
            locality by moving data from the cache of the CPU where is
            it being used to the cache of another CPU.  Hence the
            default is to do load-balancing only in the
            old-generation.  In fact, for a parallel program it is
            sometimes beneficial to disable load-balancing entirely
            with <literal>-qb</literal>.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-H</option><optional><replaceable>size</replaceable></optional>
          <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, suggested</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 0&rsqb; This option provides a
            &ldquo;suggested heap size&rdquo; for the garbage
            collector.  Think
            of <option>-H<replaceable>size</replaceable></option> as a
            variable <option>-A</option> option.  It says: I want to
            use at least <replaceable>size</replaceable> bytes, so use
            whatever is left over to increase the <option>-A</option>
            value.</para>

          <para>This option does not put
            a <emphasis>limit</emphasis> on the heap size: the heap
            may grow beyond the given size as usual.</para>

          <para>If <replaceable>size</replaceable> is omitted, then
            the garbage collector will take the size of the heap at
            the previous GC as the <replaceable>size</replaceable>.
            This has the effect of allowing for a
            larger <option>-A</option> value but without increasing
            the overall memory requirements of the program.  It can be
            useful when the default small <option>-A</option> value is
            suboptimal, as it can be in programs that create large
            amounts of long-lived data.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>-I</option><replaceable>seconds</replaceable>
	  <indexterm><primary><option>-I</option></primary>
	    <secondary>RTS option</secondary>
	  </indexterm>
	  <indexterm><primary>idle GC</primary>
	  </indexterm>
	  </term>
	<listitem>
	  <para>(default: 0.3) In the threaded and SMP versions of the RTS (see
	    <option>-threaded</option>, <xref linkend="options-linker" />), a
	    major GC is automatically performed if the runtime has been idle
	    (no Haskell computation has been running) for a period of time.
	    The amount of idle time which must pass before a GC is performed is
	    set by the <option>-I</option><replaceable>seconds</replaceable>
	    option.  Specifying <option>-I0</option> disables the idle GC.</para>

	  <para>For an interactive application, it is probably a good idea to
	    use the idle GC, because this will allow finalizers to run and
	    deadlocked threads to be detected in the idle time when no Haskell
	    computation is happening.  Also, it will mean that a GC is less
	    likely to happen when the application is busy, and so
	    responsiveness may be improved.   However, if the amount of live data in
	    the heap is particularly large, then the idle GC can cause a
	    significant delay, and too small an interval could adversely affect
	    interactive responsiveness.</para>

	  <para>This is an experimental feature, please let us know if it
	    causes problems and/or could benefit from further tuning.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
         <option>-ki</option><replaceable>size</replaceable>
         <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
         <indexterm><primary>stack, initial size</primary></indexterm>
        </term>
	<listitem>
          <para>
            &lsqb;Default: 1k&rsqb; Set the initial stack size for new
            threads.  (Note: this flag used to be
            simply <option>-k</option>, but was renamed
            to <option>-ki</option> in GHC 7.2.1.  The old name is
            still accepted for backwards compatibility, but that may
            be removed in a future version).
          </para>

          <para>
            Thread stacks (including the main thread's stack) live on
            the heap.  As the stack grows, new stack chunks are added
            as required; if the stack shrinks again, these extra stack
            chunks are reclaimed by the garbage collector.  The
            default initial stack size is deliberately small, in order
            to keep the time and space overhead for thread creation to
            a minimum, and to make it practical to spawn threads for
            even tiny pieces of work.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-kc</option><replaceable>size</replaceable>
          <indexterm><primary><option>-kc</option></primary><secondary>RTS
          option</secondary></indexterm>
          <indexterm><primary>stack</primary><secondary>chunk size</secondary></indexterm>
        </term>
        <listitem>
          <para>
            &lsqb;Default: 32k&rsqb; Set the size of &ldquo;stack
            chunks&rdquo;.  When a thread's current stack overflows, a
            new stack chunk is created and added to the thread's
            stack, until the limit set by <option>-K</option> is
            reached.
          </para>

          <para>
            The advantage of smaller stack chunks is that the garbage
            collector can avoid traversing stack chunks if they are
            known to be unmodified since the last collection, so
            reducing the chunk size means that the garbage collector
            can identify more stack as unmodified, and the GC overhead
            might be reduced.  On the other hand, making stack chunks
            too small adds some overhead as there will be more
            overflow/underflow between chunks.  The default setting of
            32k appears to be a reasonable compromise in most cases.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-kb</option><replaceable>size</replaceable>
          <indexterm><primary><option>-kc</option></primary><secondary>RTS
          option</secondary></indexterm>
          <indexterm><primary>stack</primary><secondary>chunk buffer size</secondary></indexterm>
        </term>
        <listitem>
          <para>
            &lsqb;Default: 1k&rsqb; Sets the stack chunk buffer size.
            When a stack chunk overflows and a new stack chunk is
            created, some of the data from the previous stack chunk is
            moved into the new chunk, to avoid an immediate underflow
            and repeated overflow/underflow at the boundary.  The
            amount of stack moved is set by the <option>-kb</option>
            option.
          </para>
          <para>
            Note that to avoid wasting space, this value should
            typically be less than 10&percnt; of the size of a stack
            chunk (<option>-kc</option>), because in a chain of stack
            chunks, each chunk will have a gap of unused space of this
            size.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-K</option><replaceable>size</replaceable>
          <indexterm><primary><option>-K</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>stack, maximum size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 8M&rsqb; Set the maximum stack size for
          an individual thread to <replaceable>size</replaceable>
          bytes.  If the thread attempts to exceed this limit, it will
            be send the <literal>StackOverflow</literal> exception.
          </para>
          <para>
            This option is there mainly to stop the program eating up
            all the available memory in the machine if it gets into an
            infinite loop.
          </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-m</option><replaceable>n</replaceable>
          <indexterm><primary><option>-m</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap, minimum free</primary></indexterm>
        </term>
	<listitem>
	  <para>Minimum &percnt; <replaceable>n</replaceable> of heap
          which must be available for allocation.  The default is
          3&percnt;.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-M</option><replaceable>size</replaceable>
          <indexterm><primary><option>-M</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, maximum</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: unlimited&rsqb; Set the maximum heap size to
          <replaceable>size</replaceable> bytes.  The heap normally
          grows and shrinks according to the memory requirements of
          the program.  The only reason for having this option is to
          stop the heap growing without bound and filling up all the
          available swap space, which at the least will result in the
          program being summarily killed by the operating
          system.</para>

	  <para>The maximum heap size also affects other garbage
	  collection parameters: when the amount of live data in the
	  heap exceeds a certain fraction of the maximum heap size,
	  compacting collection will be automatically enabled for the
	  oldest generation, and the <option>-F</option> parameter
	  will be reduced in order to avoid exceeding the maximum heap
	  size.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-T</option>
          <indexterm><primary><option>-T</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
        <term>
          <option>-t</option><optional><replaceable>file</replaceable></optional>
          <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<term>
          <option>-s</option><optional><replaceable>file</replaceable></optional>
          <indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<term>
          <option>-S</option><optional><replaceable>file</replaceable></optional>
          <indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<term>
          <option>--machine-readable</option>
          <indexterm><primary><option>--machine-readable</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>These options produce runtime-system statistics, such
	  as the amount of time spent executing the program and in the
	  garbage collector, the amount of memory allocated, the
	  maximum size of the heap, and so on.  The three
	  variants give different levels of detail:
          <option>-T</option> collects the data but produces no output
	  <option>-t</option> produces a single line of output in the
	  same format as GHC's <option>-Rghc-timing</option> option,
	  <option>-s</option> produces a more detailed summary at the
	  end of the program, and <option>-S</option> additionally
	  produces information about each and every garbage
	  collection.</para>

          <para>The output is placed in
          <replaceable>file</replaceable>.  If
          <replaceable>file</replaceable> is omitted, then the output
          is sent to <constant>stderr</constant>.</para>

    <para>
        If you use the <literal>-T</literal> flag then, you should
        access the statistics using
        <ulink url="&libraryBaseLocation;/GHC-Stats.html">GHC.Stats</ulink>.
    </para>

    <para>
        If you use the <literal>-t</literal> flag then, when your
        program finishes, you will see something like this:
    </para>

<programlisting>
&lt;&lt;ghc: 36169392 bytes, 69 GCs, 603392/1065272 avg/max bytes residency (2 samples), 3M in use, 0.00 INIT (0.00 elapsed), 0.02 MUT (0.02 elapsed), 0.07 GC (0.07 elapsed) :ghc&gt;&gt;
</programlisting>

    <para>
        This tells you:
    </para>

    <itemizedlist>
      <listitem>
        <para>
          The total number of bytes allocated by the program over the
          whole run.
        </para>
      </listitem>
      <listitem>
        <para>
          The total number of garbage collections performed.
        </para>
      </listitem>
      <listitem>
        <para>
          The average and maximum "residency", which is the amount of
          live data in bytes.  The runtime can only determine the
          amount of live data during a major GC, which is why the
          number of samples corresponds to the number of major GCs
          (and is usually relatively small).  To get a better picture
          of the heap profile of your program, use
          the <option>-hT</option> RTS option
          (<xref linkend="rts-profiling" />).
        </para>
      </listitem>
      <listitem>
        <para>
          The peak memory the RTS has allocated from the OS.
        </para>
      </listitem>
      <listitem>
        <para>
          The amount of CPU time and elapsed wall clock time while
          initialising the runtime system (INIT), running the program
          itself (MUT, the mutator), and garbage collecting (GC).
        </para>
      </listitem>
    </itemizedlist>

    <para>
        You can also get this in a more future-proof, machine readable
        format, with <literal>-t --machine-readable</literal>:
    </para>

<programlisting>
 [("bytes allocated", "36169392")
 ,("num_GCs", "69")
 ,("average_bytes_used", "603392")
 ,("max_bytes_used", "1065272")
 ,("num_byte_usage_samples", "2")
 ,("peak_megabytes_allocated", "3")
 ,("init_cpu_seconds", "0.00")
 ,("init_wall_seconds", "0.00")
 ,("mutator_cpu_seconds", "0.02")
 ,("mutator_wall_seconds", "0.02")
 ,("GC_cpu_seconds", "0.07")
 ,("GC_wall_seconds", "0.07")
 ]
</programlisting>

    <para>
        If you use the <literal>-s</literal> flag then, when your
        program finishes, you will see something like this (the exact
        details will vary depending on what sort of RTS you have, e.g.
        you will only see profiling data if your RTS is compiled for
        profiling):
    </para>

<programlisting>
      36,169,392 bytes allocated in the heap
       4,057,632 bytes copied during GC
       1,065,272 bytes maximum residency (2 sample(s))
          54,312 bytes maximum slop
               3 MB total memory in use (0 MB lost due to fragmentation)

  Generation 0:    67 collections,     0 parallel,  0.04s,  0.03s elapsed
  Generation 1:     2 collections,     0 parallel,  0.03s,  0.04s elapsed

  SPARKS: 359207 (557 converted, 149591 pruned)

  INIT  time    0.00s  (  0.00s elapsed)
  MUT   time    0.01s  (  0.02s elapsed)
  GC    time    0.07s  (  0.07s elapsed)
  EXIT  time    0.00s  (  0.00s elapsed)
  Total time    0.08s  (  0.09s elapsed)

  %GC time      89.5%  (75.3% elapsed)

  Alloc rate    4,520,608,923 bytes per MUT second

  Productivity  10.5% of total user, 9.1% of total elapsed
</programlisting>

    <itemizedlist>
      <listitem>
        <para>
        The "bytes allocated in the heap" is the total bytes allocated
        by the program over the whole run.
        </para>
      </listitem>
      <listitem>
        <para>
        GHC uses a copying garbage collector by default. "bytes copied
        during GC" tells you how many bytes it had to copy during
        garbage collection.
        </para>
      </listitem>
      <listitem>
        <para>
        The maximum space actually used by your program is the
        "bytes maximum residency" figure. This is only checked during
        major garbage collections, so it is only an approximation;
        the number of samples tells you how many times it is checked.
        </para>
      </listitem>
      <listitem>
        <para>
        The "bytes maximum slop" tells you the most space that is ever
        wasted due to the way GHC allocates memory in blocks.  Slop is
        memory at the end of a block that was wasted.  There's no way
        to control this; we just like to see how much memory is being
        lost this way.
        </para>
      </listitem>
      <listitem>
        <para>
        The "total memory in use" tells you the peak memory the RTS has
        allocated from the OS.
        </para>
      </listitem>
      <listitem>
        <para>
        Next there is information about the garbage collections done.
        For each generation it says how many garbage collections were
        done, how many of those collections were done in parallel,
        the total CPU time used for garbage collecting that generation,
        and the total wall clock time elapsed while garbage collecting
        that generation.
        </para>
      </listitem>
      <listitem>
        <para>The <literal>SPARKS</literal> statistic refers to the
          use of <literal>Control.Parallel.par</literal> and related
          functionality in the program.  Each spark represents a call
          to <literal>par</literal>; a spark is "converted" when it is
          executed in parallel; and a spark is "pruned" when it is
          found to be already evaluated and is discarded from the pool
          by the garbage collector.  Any remaining sparks are
          discarded at the end of execution, so "converted" plus
          "pruned" does not necessarily add up to the total.</para>
      </listitem>
      <listitem>
        <para>
        Next there is the CPU time and wall clock time elapsed broken
        down by what the runtime system was doing at the time.
        INIT is the runtime system initialisation.
        MUT is the mutator time, i.e. the time spent actually running
        your code.
        GC is the time spent doing garbage collection.
        RP is the time spent doing retainer profiling.
        PROF is the time spent doing other profiling.
        EXIT is the runtime system shutdown time.
        And finally, Total is, of course, the total.
        </para>
        <para>
        %GC time tells you what percentage GC is of Total.
        "Alloc rate" tells you the "bytes allocated in the heap" divided
        by the MUT CPU time.
        "Productivity" tells you what percentage of the Total CPU and wall
        clock elapsed times are spent in the mutator (MUT).
        </para>
      </listitem>
    </itemizedlist>

    <para>
        The <literal>-S</literal> flag, as well as giving the same
        output as the <literal>-s</literal> flag, prints information
        about each GC as it happens:
    </para>

<programlisting>
    Alloc    Copied     Live    GC    GC     TOT     TOT  Page Flts
    bytes     bytes     bytes  user  elap    user    elap
   528496     47728    141512  0.01  0.02    0.02    0.02    0    0  (Gen:  1)
[...]
   524944    175944   1726384  0.00  0.00    0.08    0.11    0    0  (Gen:  0)
</programlisting>

    <para>
        For each garbage collection, we print:
    </para>

    <itemizedlist>
      <listitem>
        <para>
          How many bytes we allocated this garbage collection.
        </para>
      </listitem>
      <listitem>
        <para>
          How many bytes we copied this garbage collection.
        </para>
      </listitem>
      <listitem>
        <para>
          How many bytes are currently live.
        </para>
      </listitem>
      <listitem>
        <para>
          How long this garbage collection took (CPU time and elapsed
          wall clock time).
        </para>
      </listitem>
      <listitem>
        <para>
          How long the program has been running (CPU time and elapsed
          wall clock time).
        </para>
      </listitem>
      <listitem>
        <para>
          How many page faults occurred this garbage collection.
        </para>
      </listitem>
      <listitem>
        <para>
          How many page faults occurred since the end of the last garbage
          collection.
        </para>
      </listitem>
      <listitem>
        <para>
          Which generation is being garbage collected.
        </para>
      </listitem>
    </itemizedlist>

	</listitem>
      </varlistentry>
    </variablelist>

  </sect2>

  <sect2>
    <title>RTS options for concurrency and parallelism</title>

    <para>The RTS options related to concurrency are described in
      <xref linkend="using-concurrent" />, and those for parallelism in
      <xref linkend="parallel-options"/>.</para>
  </sect2>

  <sect2 id="rts-profiling">
    <title>RTS options for profiling</title>

    <para>Most profiling runtime options are only available when you
    compile your program for profiling (see
    <xref linkend="prof-compiler-options" />, and
    <xref linkend="rts-options-heap-prof" /> for the runtime options).
    However, there is one profiling option that is available
    for ordinary non-profiled executables:</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-hT</option>
          <indexterm><primary><option>-hT</option></primary><secondary>RTS
              option</secondary></indexterm>
        </term>
        <listitem>
          <para>(can be shortened to <option>-h</option>.) Generates a basic heap profile, in the
            file <literal><replaceable>prog</replaceable>.hp</literal>.
            To produce the heap profile graph,
            use <command>hp2ps</command> (see <xref linkend="hp2ps"
                                                    />).  The basic heap profile is broken down by data
            constructor, with other types of closures (functions, thunks,
            etc.) grouped into broad categories
            (e.g. <literal>FUN</literal>, <literal>THUNK</literal>).  To
            get a more detailed profile, use the full profiling
            support (<xref linkend="profiling" />).</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </sect2>

  <sect2 id="rts-eventlog">
    <title>Tracing</title>

    <indexterm><primary>tracing</primary></indexterm>
    <indexterm><primary>events</primary></indexterm>
    <indexterm><primary>eventlog files</primary></indexterm>

    <para>
      When the program is linked with the <option>-eventlog</option>
      option (<xref linkend="options-linker" />), runtime events can
      be logged in two ways:
    </para>

    <itemizedlist>
      <listitem>
        <para>
          In binary format to a file for later analysis by a
          variety of tools.  One such tool
          is <ulink url="http://www.haskell.org/haskellwiki/ThreadScope">ThreadScope</ulink><indexterm><primary>ThreadScope</primary></indexterm>,
          which interprets the event log to produce a visual parallel
          execution profile of the program.
        </para>
      </listitem>
      <listitem>
        <para>
          As text to standard output, for debugging purposes.
        </para>
      </listitem>
    </itemizedlist>

    <variablelist>
      <varlistentry>
        <term>
          <option>-l<optional><replaceable>flags</replaceable></optional></option>
          <indexterm><primary><option>-l</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
        <listitem>
          <para>
            Log events in binary format to the
            file <filename><replaceable>program</replaceable>.eventlog</filename>.
            Without any <replaceable>flags</replaceable> specified, this logs a
            default set of events, suitable for use with tools like ThreadScope.
          </para>

          <para>
            For some special use cases you may want more control over which
            events are included. The <replaceable>flags</replaceable> is a
            sequence of zero or more characters indicating which classes of
            events to log. Currently these the classes of events that can
            be enabled/disabled:
            <simplelist>
              <member>
                <option>s</option> &#8212; scheduler events, including Haskell
                thread creation and start/stop events. Enabled by default.
              </member>
              <member>
                <option>g</option> &#8212; GC events, including GC start/stop.
                Enabled by default.
              </member>
              <member>
                <option>p</option> &#8212; parallel sparks (sampled).
                Enabled by default.
              </member>
              <member>
                <option>f</option> &#8212; parallel sparks (fully accurate).
                Disabled by default.
              </member>
              <member>
                <option>u</option> &#8212; user events. These are events emitted
                from Haskell code using functions such as 
                <literal>Debug.Trace.traceEvent</literal>. Enabled by default.
              </member>
            </simplelist>
          </para>

          <para>            
            You can disable specific classes, or enable/disable all classes at
            once:
            <simplelist>
              <member>
                <option>a</option> &#8212; enable all event classes listed above
              </member>
              <member>
                <option>-<replaceable>x</replaceable></option> &#8212; disable the
                given class of events, for any event class listed above or
                <option>-a</option> for all classes
              </member>
            </simplelist>
            For example, <option>-l-ag</option> would disable all event classes
            (<option>-a</option>) except for GC events (<option>g</option>).
          </para>

          <para>            
            For spark events there are two modes: sampled and fully accurate.
            There are various events in the life cycle of each spark, usually
            just creating and running, but there are some more exceptional
            possibilities. In the sampled mode the number of occurrences of each
            kind of spark event is sampled at frequent intervals. In the fully
            accurate mode every spark event is logged individually. The latter
            has a higher runtime overhead and is not enabled by default.
          </para>

          <para>
            The format of the log file is described by the header
            <filename>EventLogFormat.h</filename> that comes with
            GHC, and it can be parsed in Haskell using
            the <ulink url="http://hackage.haskell.org/package/ghc-events">ghc-events</ulink>
            library.  To dump the contents of
            a <literal>.eventlog</literal> file as text, use the
            tool <literal>ghc-events show</literal> that comes with
            the <ulink url="http://hackage.haskell.org/package/ghc-events">ghc-events</ulink>
            package.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-v</option><optional><replaceable>flags</replaceable></optional>
          <indexterm><primary><option>-v</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
        <listitem>
          <para>
            Log events as text to standard output, instead of to
            the <literal>.eventlog</literal> file.
            The <replaceable>flags</replaceable> are the same as
            for <option>-l</option>, with the additional
            option <literal>t</literal> which indicates that the
            each event printed should be preceded by a timestamp value
            (in the binary <literal>.eventlog</literal> file, all
            events are automatically associated with a timestamp).
          </para>
        </listitem>
      </varlistentry>

    </variablelist>

    <para>
      The debugging
      options <option>-D<replaceable>x</replaceable></option> also
      generate events which are logged using the tracing framework.
      By default those events are dumped as text to stdout
      (<option>-D<replaceable>x</replaceable></option>
      implies <option>-v</option>), but they may instead be stored in
      the binary eventlog file by using the <option>-l</option>
      option.
    </para>
  </sect2>

  <sect2 id="rts-options-debugging">
    <title>RTS options for hackers, debuggers, and over-interested
    souls</title>

    <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>

    <para>These RTS options might be used (a)&nbsp;to avoid a GHC bug,
    (b)&nbsp;to see &ldquo;what's really happening&rdquo;, or
    (c)&nbsp;because you feel like it.  Not recommended for everyday
    use!</para>

    <variablelist>

      <varlistentry>
	<term>
          <option>-B</option>
          <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>Sound the bell at the start of each (major) garbage
	  collection.</para>

	  <para>Oddly enough, people really do use this option!  Our
          pal in Durham (England), Paul Callaghan, writes: &ldquo;Some
          people here use it for a variety of
          purposes&mdash;honestly!&mdash;e.g., confirmation that the
          code/machine is doing something, infinite loop detection,
          gauging cost of recently added code. Certain people can even
          tell what stage &lsqb;the program&rsqb; is in by the beep
          pattern. But the major use is for annoying others in the
          same office&hellip;&rdquo;</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-D</option><replaceable>x</replaceable>
          <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>
            An RTS debugging flag; only available if the program was
	    linked with the <option>-debug</option> option.  Various
	    values of <replaceable>x</replaceable> are provided to
	    enable debug messages and additional runtime sanity checks
	    in different subsystems in the RTS, for
	    example <literal>+RTS -Ds -RTS</literal> enables debug
	    messages from the scheduler.
	    Use <literal>+RTS&nbsp;-?</literal> to find out which
	    debug flags are supported.
          </para>

          <para>
            Debug messages will be sent to the binary event log file
            instead of stdout if the <option>-l</option> option is
            added.  This might be useful for reducing the overhead of
            debug tracing.
          </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-r</option><replaceable>file</replaceable>
          <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>ticky ticky profiling</primary></indexterm>
          <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
        </term>
	<listitem>
	  <para>Produce &ldquo;ticky-ticky&rdquo; statistics at the
          end of the program run (only available if the program was
          linked with <option>-debug</option>).
          The <replaceable>file</replaceable> business works just like
          on the <option>-S</option> RTS option, above.</para>

          <para>For more information on ticky-ticky profiling, see
          <xref linkend="ticky-ticky"/>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-xc</option>
          <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>(Only available when the program is compiled for
	  profiling.)  When an exception is raised in the program,
          this option causes a stack trace to be
          dumped to <literal>stderr</literal>.</para>

	  <para>This can be particularly useful for debugging: if your
	  program is complaining about a <literal>head []</literal>
	  error and you haven't got a clue which bit of code is
	  causing it, compiling with <literal>-prof
          -fprof-auto</literal> and running with <literal>+RTS -xc
	  -RTS</literal> will tell you exactly the call stack at the
	  point the error was raised.</para>

          <para>The output contains one report for each exception
          raised in the program (the program might raise and catch
          several exceptions during its execution), where each report
          looks something like this:
          </para>

<screen>
*** Exception raised (reporting due to +RTS -xc), stack trace:
  GHC.List.CAF
  --> evaluated by: Main.polynomial.table_search,
  called from Main.polynomial.theta_index,
  called from Main.polynomial,
  called from Main.zonal_pressure,
  called from Main.make_pressure.p,
  called from Main.make_pressure,
  called from Main.compute_initial_state.p,
  called from Main.compute_initial_state,
  called from Main.CAF
  ...
</screen>
          <para>The stack trace may often begin with something
          uninformative like <literal>GHC.List.CAF</literal>; this is
          an artifact of GHC's optimiser, which lifts out exceptions
          to the top-level where the profiling system assigns them to
          the cost centre "CAF".  However, <literal>+RTS -xc</literal>
          doesn't just print the current stack, it looks deeper and
          reports the stack at the time the CAF was evaluated, and it
          may report further stacks until a non-CAF stack is found.  In
          the example above, the next stack (after <literal>-->
          evaluated by</literal>) contains plenty of information about
          what the program was doing when it evaluated <literal>head
          []</literal>.</para>

          <para>Implementation details aside, the function names in
          the stack should hopefully give you enough clues to track
          down the bug.</para>

          <para>
            See also the function <literal>traceStack</literal> in the
            module <literal>Debug.Trace</literal> for another way to
            view call stacks.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-Z</option>
          <indexterm><primary><option>-Z</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>Turn <emphasis>off</emphasis> &ldquo;update-frame
          squeezing&rdquo; at garbage-collection time.  (There's no
          particularly good reason to turn it off, except to ensure
          the accuracy of certain data collected regarding thunk entry
          counts.)</para>
	</listitem>
      </varlistentry>
    </variablelist>

  </sect2>

  <sect2 id="ghc-info">
    <title>Getting information about the RTS</title>

    <indexterm><primary>RTS</primary></indexterm>

    <para>It is possible to ask the RTS to give some information about
    itself. To do this, use the <option>--info</option> flag, e.g.</para>
<screen>
$ ./a.out +RTS --info
 [("GHC RTS", "YES")
 ,("GHC version", "6.7")
 ,("RTS way", "rts_p")
 ,("Host platform", "x86_64-unknown-linux")
 ,("Host architecture", "x86_64")
 ,("Host OS", "linux")
 ,("Host vendor", "unknown")
 ,("Build platform", "x86_64-unknown-linux")
 ,("Build architecture", "x86_64")
 ,("Build OS", "linux")
 ,("Build vendor", "unknown")
 ,("Target platform", "x86_64-unknown-linux")
 ,("Target architecture", "x86_64")
 ,("Target OS", "linux")
 ,("Target vendor", "unknown")
 ,("Word size", "64")
 ,("Compiler unregisterised", "NO")
 ,("Tables next to code", "YES")
 ]
</screen>
    <para>The information is formatted such that it can be read as a
    of type <literal>[(String, String)]</literal>. Currently the following
    fields are present:</para>

    <variablelist>

      <varlistentry>
        <term><literal>GHC RTS</literal></term>
        <listitem>
          <para>Is this program linked against the GHC RTS? (always
          "YES").</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>GHC version</literal></term>
        <listitem>
          <para>The version of GHC used to compile this program.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>RTS way</literal></term>
        <listitem>
          <para>The variant (&ldquo;way&rdquo;) of the runtime. The
          most common values are <literal>rts</literal> (vanilla),
          <literal>rts_thr</literal> (threaded runtime, i.e. linked using the
          <literal>-threaded</literal> option) and <literal>rts_p</literal>
          (profiling runtime, i.e. linked using the <literal>-prof</literal>
          option). Other variants include <literal>debug</literal>
          (linked using <literal>-debug</literal>),
          <literal>t</literal> (ticky-ticky profiling) and
          <literal>dyn</literal> (the RTS is
          linked in dynamically, i.e. a shared library, rather than statically
          linked into the executable itself). These can be combined,
          e.g. you might have <literal>rts_thr_debug_p</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
            <literal>Target platform</literal>,
            <literal>Target architecture</literal>,
            <literal>Target OS</literal>,
            <literal>Target vendor</literal>
        </term>
        <listitem>
          <para>These are the platform the program is compiled to run on.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
            <literal>Build platform</literal>,
            <literal>Build architecture</literal>,
            <literal>Build OS</literal>,
            <literal>Build vendor</literal>
        </term>
        <listitem>
          <para>These are the platform where the program was built
          on. (That is, the target platform of GHC itself.) Ordinarily
          this is identical to the target platform. (It could potentially
          be different if cross-compiling.)</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
            <literal>Host platform</literal>,
            <literal>Host architecture</literal>
            <literal>Host OS</literal>
            <literal>Host vendor</literal>
        </term>
        <listitem>
          <para>These are the platform where GHC itself was compiled.
          Again, this would normally be identical to the build and
          target platforms.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>Word size</literal></term>
        <listitem>
          <para>Either <literal>"32"</literal> or <literal>"64"</literal>,
          reflecting the word size of the target platform.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>Compiler unregistered</literal></term>
        <listitem>
          <para>Was this program compiled with an
          <link linkend="unreg">&ldquo;unregistered&rdquo;</link>
          version of GHC? (I.e., a version of GHC that has no platform-specific
          optimisations compiled in, usually because this is a currently
          unsupported platform.) This value will usually be no, unless you're
          using an experimental build of GHC.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>Tables next to code</literal></term>
        <listitem>
          <para>Putting info tables directly next to entry code is a useful
          performance optimisation that is not available on all platforms.
          This field tells you whether the program has been compiled with
          this optimisation. (Usually yes, except on unusual platforms.)</para>
        </listitem>
      </varlistentry>

    </variablelist>

  </sect2>
</sect1>

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