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README.mpitrace

This README describes libraries that can be used to gather MPI timining
data, hardware counter data, and other performance measurements.  The
MPI measurements use the "PMPI" profiling interface:  for each MPI
routine, there is a wrapper that collects timing data, and calls the
equivalent PMPI routine:

int MPI_Send(...) {
  start_timing();
  PMPI_Send(...);
  stop_timing();
  log_the_event();
}

There is one combined wrapper-set for apps that use Fortran and C:

libmpitrace.a   : wrappers for MPI only  

libmpihpm.a     : wrappers for MPI plus hardware counters for pure
                  MPI applications

libmpihpm_smp.a : wrappers for MPI plus hardware counters for mixed
                  MPI + OpenMP applications

The libmpihpm.a library is the same as the libmpitrace.a library,
with the addition that hardware counters are started in the wrapper
for MPI_Init() and stopped in the wrapper for MPI_Finalize().  It is
also possible to monitor a specific code section with counters, or
just for MPI information.  See the section on hardware counters for
more information on libmpihpm.a and libmpihpm_smp.a


The wrappers can be used in two modes.  The default mode is to collect
only a timing summary.  The other mode is to collect both a timing
summary and a time-history of MPI calls suitable for graphical display.
To enable this event-tracing method from the beginning of the program, 
you can set an environment variable:

export TRACE_ALL_EVENTS=yes

This will save a record of all MPI events after MPI_Init() until the
application completes, or until the limited trace buffer is full.
For better control, one can put calls in the code to start/stop
event tracing for selected code blocks.  For more information on event 
tracing, see the Event Tracing section.


In summary mode (the default) you normally get just the total elapsed
time in each MPI routine, the total "communication" time, and some info
on message-size distributions.	You can get MPI profiling information
that associates elapsed time in MPI routines with instruction address
or call site in the application by setting an environment variable:

export PROFILE_BY_CALL_SITE=yes

This option adds some overhead because it has to do a traceback to
identify the code location for each MPI call, but it provides some extra
information that can be very useful.  In some cases there may be nested
layers on top of MPI, and you may need to profile higher up the call 
chain.  You can do this by setting another environment variable.  For 
example, setting TRACEBACK_LEVEL=2 tells the library to save addresses 
starting not with the location of the MPI call (level = 1), but from the
parent in the call chain (level = 2).	To use this profiling option, 
compile and link with -g; and then use the addr2line utility to 
translate from instruction address to source file and line number.

Also, in summary mode, the default is to save just the summary data
from MPI rank 0, and the MPI ranks with the min, max, and median
communication times.  That way the number of text files generated
remains small even for very large process counts.  The file for MPI
rank 0 has a collection of some of the basic data from every rank,
so there is not much information lost.  Also, if you use the -pg
profiling option, the same output filter is applied to the generation
of gmon.out files : by default you will only get gmon.out files for
MPI rank 0 and the ranks with the min, max, and median communication
times.  If you want all gmon.out files and/or all MPI summary files,
you can set SAVE_ALL_TASKS=yes as an env variable at runtime.  Also,
for added flexibility, you can define a list of MPI ranks that will
be used to save detailed output, set the env variable SAVE_LIST to
a comma-separated list of MPI ranks in MPI_COMM_WORLD; for example:
export SAVE_LIST="0,10,20,30,40,50".


In either mode there is an option to collect information about the 
number of hops for point-to-point communication on the torus network.
This feature can be enabled by setting an environment variable:

export TRACE_SEND_PATTERN=yes

When this variable is set, the wrappers keep track of how many bytes are
sent to each task, and a matrix is written during MPI_Finalize which 
lists how many bytes were sent from each task to all other tasks.  This 
matrix can be used as input to external utilities that can generate 
efficient mappings of MPI tasks onto torus coordinates.  The wrappers 
also provide the average number of hops for all flavors of MPI_Send.  
The wrappers do not track the message-traffic patterns in collective 
calls, such as MPI_Alltoall.  Only point-to-point send operations are 
tracked.  In addition to the send_bytes matrix, there are pattern files
written, listing the destination ranks and the location and number of
bytes sent to each destination rank.

In summary mode, the time spent in MPI routines is accumulated starting
in MPI_Init() and stopping in MPI_Finalize().  In some cases you may want
to collect MPI timing data for only a certain interval.  You can do this
by calling routines to start/stop the collection of summary statistics:

Fortran syntax:
        call summary_start()
        do work + MPI ...
        call summary_stop()

C syntax:
  summary_start();
  do work + MPI ...
  summary_stop();

If you control summary statistics in this way, the accumulated data and
timers are reset when the first call to summary_start() is made, and
the output files should contain MPI timing data for just the interval
that you specified.

-----------------------------------------------------------------------

How to Use:

To use one of these libraries, just add the trace library after the
user's .o and .a files, but before the MPI library in the link order.
This amounts to a small change in the makefile for the linking step.
Run the application as you normally would.  The wrapper for MPI_Finalize()
writes the timing summaries in files called mpi_profile.*.  In this
version of the wrappers, these text files have names:

mpi_profile.jobid.rank

where rank is the MPI rank in MPI_COMM_WORLD.  The jobid was added to
the name of the mpi_profile to ensure a unique name for each job.  The 
mpi_profile for rank 0 is special: it contains a timing summary from each 
task.  Detailed information is saved from the ranks with the minimum, 
maximum, and median times spent in MPI calls, by defualt, but you can
control output as described above.

-----------------------------------------------------------------------

Hardware Counters:

With the libmpihpm.a library, you get hardware counter data in addition 
to MPI information.  The wrapper for MPI_Finalize() writes one extra 
file for each MPI rank that is saving detailed data:

hpm_process_summary.jobid.rank  (text file with a readable summary)

and it writes one file with aggregate counts for each process in the
job :

hpm_job_summary.jobid.rank

There are two libraries that include hardware counters: libmpihpm.a, 
and libmpihpm_smp.a.  The libmpihpm.a library is intended for pure MPI
applications, and the libmpihpm_smp.a is intended for applications 
that mix OpenMP and MPI. In both cases, the MPI wrappers call 
HPM_Start("mpiAll") in MPI_Init() and they call HPM_Stop("mpiAll") in 
MPI_Finalize; so you should always get counter data for the "mpiAll" 
code-block.  You can also add calls to HPM_Start/HPM_Stop as described 
below to collect data on specific code blocks.

Fortran syntax:
      call hpm_start('myblock')
      do work ...
      call hpm_stop('myblock')

C syntax:
  HPM_Start("myblock");
  do work ...
  HPM_Stop("myblock");

C prototypes:
 void HPM_Start(char *);
 void HPM_Stop(char *);

C++ prototypes:
 extern "C" void HPM_Start(char *);
 extern "C" void HPM_Stop(char *);

To use the libraries that have hardware counters, you need to link
with the appropriate system libraries:

/bgusr/walkup/mpi_trace/bgq/lib/libmpihpm.a \
/bgsys/drivers/ppcfloor/bgpm/lib/libbgpm.a \
 -L/bgsys/drivers/ppcfloor/spi/lib -lSPI_upci_cnk

and add -qsmp if using libmpihpm_smp.a.

BG/Q has counters that are shared across the node for things like
memory and network resources.  If you want to interpret the shared
counters, it is best if all processes on a given node make the same
sequence of HPM_Start()/HPM_Stop() calls.  The default setting is
for counter data to be collected and aggregated over all of the 
processes that share a node.  That corresponds to HPM_SCOPE=node.
You can also set HPM_SCOPE=process, which will just report counts
per MPI process, without performing node-aggregation or computing
derived metrics.  The default counter group is HPM_GROUP=0, which
provides a lot of basic information.  Additional counter groups can
be defined ... see hpm.c.

-----------------------------------------------------------------------

Output Location:

Normally the mpi_profile files and hpm files are written in the
working directory.  You can specify an alternative location by setting
the env variable TRACE_DIR to a directory where you have write 
permission.

-----------------------------------------------------------------------

Event Tracing:

You can save a time-stamped record of each MPI call by setting an 
environment variable:

export TRACE_ALL_EVENTS=yes

This will save a record of all MPI events after MPI_Init() until the
application completes, or until the limited trace buffer is full.
For better control, you can put calls in the code to start/stop
tracing around specific code blocks:

Fortran syntax:
        call trace_start()
        do work + mpi ...
        call trace_stop()

C syntax:
  void trace_start(void);
  void trace_stop(void);

  trace_start();
  do work + mpi ...
  trace_stop();

C++ syntax:
  extern "C" void trace_start(void);
  extern "C" void trace_stop(void);

  trace_start();
  do work + mpi ...
  trace_stop();

When event tracing is enabled, the wrappers save a time-stamped record
of every MPI call for graphical display.  This adds some overhead,
about 1-2 microseconds per call.  The event-tracing method uses a small
buffer in memory - up to 5*10**4 events per task - and so this is best
suited for short-running applications, or time-stepping codes for just a
few steps. The "traceview" utility can be used to display the tracefile
recorded with event-tracing mode.  To use the event-tracing mode, add 
-g for both compilation and linking - this is needed to identify the 
source-file and line-number for each event.  The default trace buffer
is enough for 50000 MPI calls, using 48 bytes per call.  You can set
an environment variable to control the trace buffer size:

export TRACE_BUFFER_SIZE=10000000 (for 10**7 bytes)

however, remember that this buffer is in memory, which may be a
constrained resource.

When saving MPI event records, it is easy to generate trace files that
are just too large to visualize.  To cut down on the data volume, the
default behavior when you set TRACE_ALL_EVENTS=yes is to save event
records from MPI tasks 0-255, or for all MPI processes if there are 256
or fewer processes in MPI_COMM_WORLD.  That should be enough to provide
a good visual record of the communication pattern.  If you want to save
data from all tasks, you have to set TRACE_ALL_TASKS=yes.  To provide
more control, you can set TRACE_MAX_RANK=#.  For example, if you set
TRACE_MAX_RANK=2048, you will get trace data from 2048 tasks, 0-2047,
provided you actually have at least 2048 tasks in your job.  By using
the time-stamped trace feature selectively, both in time (trace_start/
trace_stop), and by MPI rank, you can get good insight into the MPI
performance of very large complex parallel applications.

Some applications call certain MPI routines very frequently, which can
cause trace buffers to overflow very quickly.  You can turn off tracing
for such routines to keep trace files manageable.  For example, you can
set an env variable:

export TRACE_DISABLE_LIST="MPI_Comm_rank,MPI_Iprobe"
or in mpirun: -env "TRACE_DISABLE_LIST=MPI_Comm_rank,MPI_Iprobe"

to disable tracing for MPI_Comm_rank() and MPI_Iprobe().  You can specify
the list using a comma to separate the items in the list, and the test is
not sensitive to the case (upper/lower).  The listed routines will still 
be profiled in the timing summary, but they will be excluded from the 
events trace file.

Trace records contain the starting and ending time for each MPI call and
also the parent and grandparent instruction addresses.	You can associate
instruction addresses with source-file and line-number information using
the addr2line utility:

addr2line -e your.x  hex_instruction_address

The -g option can be used along with optimization, but sometimes the
actual parent or grandparent may be off a line or two - still adequate to
completly identify the event.  The "parent address" is the return address
after the MPI call.  This is typically the next executable statement
after the called MPI routine, so don't expect a perfect line-by-line
source-code association with your MPI calls.

-----------------------------------------------------------------------

Profiling:

The timing libraries provide some support for several profiling methods.
If you use standard Unix profiling (-pg option), the libraries will
automatically save the gmon.out files from only rank 0, and the ranks
that had the min, max, and median times in MPI.  That prevents the 
system from generating one gmon.out file per MPI rank, which is to be
avoided when the number of MPI ranks is very large.  The methods that
are used to control MPI profiling output, SAVE_LIST, and SAVE_ALL_TASKS,
also apply to other profiling outputs.

In libmpitrace.a and libmpihpm.a there is experimental support for
Vprof.  You can set an environment variable VPROF_PROFILE=yes to enable
Vprof-profiling from MPI_Init() to MPI_Finalize().  Alternativly, you
can add calls to vprof_start()/vprof_stop() around a specific code block
that you want to profile. This is a change from the standard vprof 
package, which used vmon_begin()/vmon_done().  The reason for the change
is that vmon_done() was designed to both stop profiling and write the
output file, but it is more convenient to separate those functions.  
With vprof_start()/vprof_stop() the "stop" routine just stops profiling.
The file write is handled in the wrapper for MPI_Finalize().  That way
it is possible to use the same output filter that is used for the other
profiling functions (default is output from MPI rank 0 and the ranks
with min, max, and median times in MPI).  The vprof GUI interface and 
the command-line utility "cprof" can be built for the BG/Q front-end for
analysis of the vmon.out files. Typical use would be: 

cprof -e -n your.exe vmon.out.rank >cprofile.rank &

For historical reasons, there is an additional function-level profiling
feature.  This was added for BG/L in the years before BG/L supported
profiling with -pg.  The idea was to get the compiler to automatically
instrument entry/exit for each compiled routine.  To use this feature, 
you must use XL compilers and compile with the option:

-qdebug=function_trace

A simple "flat profile" is obtained, by starting an elapsed-time timer
upon entry, and stopping the timer upon exit for each routine compiled 
with -qdebug=function_trace. To enable the feature, add the environment 
variable:

FLAT_PROFILE=yes

when you submit your job.  This feature also turns on memory tracking,
and a report of memory utilization is printed in the wrapper for 
MPI_Finalize().  This feature works best for codes that do not make
frequent function calls, and for single-threaded applications.  The
overhead is prohibitive for applications that make frequent function
calls.  It is recommended to use either -pg or the vprof profiling 
method.

-----------------------------------------------------------------------

Changes:

1. March 19,2004.  Changed the basic event structure to 48 bytes to make
   event structures a multiple of sizeof(double).  This solves problems
   with structure alignment on different architectures. Both the trace
   libraries and "traceview" need to be updated to work correctly with
   the larger event structures.

2. June 15, 2004.  Added a feature to monitor the pattern of point-to-
   point communication on the torus.  This can be enabled by setting
   TRACE_SEND_PATTERN=yes as an environment variable.  Also changed
   the clock frequency from hard-coded to a variable obtained from the
   BG/L personality.

3. July 19, 2004.  Added TRACEBACK_ERRORS env variable.  If this is set
   to "yes", you get a traceback with any MPI error.  The traceback
   lists instruction addresses, so use addr2line to get source file and
   line number.

4. January 27, 2004.  Changed the default behavior to save data from a
   maximum of 256 MPI processes (0-255), when event tracing is enabled.
   Added env variable "TRACE_ALL_TASKS=yes" if you really want traces
   from all tasks; and "MAX_TRACE_RANK=#" is you only want to save data
   from MPI ranks 0, ..., #-1.

5. March 7, 2005.  Added a first version of function profiling.  Add the
   env variable FLAT_PROFILE=yes, and compile all routines to be profiled
   with -qdebug=function_trace; and you should get flat profiles at the
   end, when MPI_Finalize is called.  The method uses elapsed-time timers
   around entry and exit of each routine, and thus adds some overhead.

6. May 18, 2005.  Added code to correctly track point-to-point send
   operations for arbitrary communicators.  Also the number of hops
   should now be computed correctly for both torus and mesh topologies.
   These changes affect jobs that specify TRACE_SEND_PATTERN=yes.

7. May 26, 2005.  Added memory-tracking code to the function_trace
   routines.  When the code is compiled with -qdebug=function_trace,
   and FLAT_PROFILE=yes is set in the run script, there will be both
   routine-level timing data and memory utilization statistics printed
   when MPI_Finalize is called.

8. June 23, 2005. Added bgl_perfctr() option to start hardware counters
   in MPI_Init() and stop them in MPI_Finalize().  The HPM enabled
   wrappers are in libmpihpm_c/f.a, and to use them you must also link
   with the bgl performance-counter library: -lbgl_perfctr.rts.

9. August 30, 2005.  Fixed bgl_perfctr() to work in virtual node mode.

10.February 7, 2006.  Added profiling by the calling point.  If you
   set an env variable PROFILE_BY_CALLER=yes, you get a summary of
   MPI elapsed times listed with the instruction address in the app.
   This should make it possible to identify where in the code the MPI
   time is comming from.  You need to use addr2line to translate from
   instruction address to source file and line number, so -g is required
   when compiling and linking if you want to use this option.

11.March 24, 2006.  Changed to a single library for Fortran, C/C++, or
   any mixture, due to changes in the BG/L MPI layer.  No longer need
   separate libs for different languages.

12.April 5, 2006. Added a feature to start/stop the collection of
   cumulative MPI timing, by adding calls to the code.  The default
   remains to start timing in MPI_Init and stop in MPI_Finalize. To
   collect statistics for just a selected code block, insert calls 
   to summary_start()/summary_stop().
   
13.Sept. 19, 2006.  Added an option to save output from all tasks:
   set env variable SAVE_ALL_TASKS=yes.  This option will result in
   one summary output file from each MPI rank, and it will preserve
   the gmon.out file from each MPI rank.  This can result in a very
   large amount of data generated upon program termination, so the
   default behavior remains to save only output from MPI ranks 0, and
   the ranks with the min, max, and median communication times.
  
14.January 22, 2007.  Added entry points for MPI-IO routines.  The
   "communication time" is still the same, cumulative over MPI 1.2
   message-passing functions; the time spent in MPI-IO is reported
   separately.

15.March 2007.  Fixed code to profile by call site, and changed the
   env variable from old PROFILE_BY_CALLER to PROFILE_BY_CALL_SITE
   for compatibility with Linux and AIX versions.  For event tracing,
   the trace buffer size (in bytes) can now be set by an env variable:
   TRACE_BUFFER_SIZE=4800000 for example.  Each event record takes 48
   bytes, and the default size is 30000 events, or 1440000 bytes.

16.May 2008.  Fixed BGP hardware counter code, and added BGP_STATS
   and BGP_STATS_INTERVAL env variables for an experimental way to
   get "real-time" statistics from running jobs.

17.November 2008.  Added env variable TRACE_DISABLE_LIST which can
   be used to turn-off event tracing for certain MPI routines.  This
   can help keep trace files manageable when the application calls
   certain routines (MPI_Iprobe or MPI_Comm_rank) too frequently.

18.November 2008.  Added the ability to start/stop the BGP counters

 ...


21.December 2009.  Added automatic pattern-file generation when the
   env variable TRACE_SEND_PATTERN=yes.  Changed to a sparse-matrix
   format for the send_bytes matrix:
      -------------------------
      num_ranks (4 bytes)
      -------------------------
      num_partners (4 bytes)
      array of [bytes(float), dest_rank(int)] of size num_partners
      -------------------------
      repeat for every rank, just keeping terms with bytes > 0.
      
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