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Ddoc
$(SPEC_S Garbage Collection,
$(P D is a fully garbage collected language. That means that it is never
necessary
to free memory. Just allocate as needed, and the garbage collector will
periodically return all unused memory to the pool of available memory.
)
$(P C and C++ programmers accustomed to explicitly managing memory
allocation and
deallocation will likely be skeptical of the benefits and efficacy of
garbage collection. Experience both with new projects written with
garbage collection in mind, and converting existing projects to garbage
collection shows that:
)
$(UL
$(LI Garbage collected programs are faster. This is counterintuitive,
but the reasons are:
$(UL
$(LI Reference counting is a common solution to solve explicit
memory allocation problems. The code to implement the increment and
decrement operations whenever assignments are made is one source
of slowdown. Hiding it behind smart pointer classes doesn't help
the speed. (Reference counting methods are not a general solution
anyway, as circular references never get deleted.)
)
$(LI Destructors are used to deallocate resources acquired by an object.
For most classes, this resource is allocated memory.
With garbage collection, most destructors then become empty and
can be discarded entirely.
)
$(LI All those destructors freeing memory can become significant when
objects are allocated on the stack. For each one, some mechanism must
be established so that if an exception happens, the destructors all
get called in each frame to release any memory they hold. If the
destructors become irrelevant, then there's no need to set up special
stack frames to handle exceptions, and the code runs faster.
)
$(LI All the code necessary to manage memory can add up to quite a
bit. The larger a program is, the less in the cache it is,
the more paging it does, and the slower
it runs.
)
$(LI Garbage collection kicks in only when memory gets tight. When
memory is not tight, the program runs at full speed and does not
spend any time freeing memory.
)
$(LI Modern garbage collectors are far more advanced now than the
older, slower ones. Generational, copying collectors eliminate much
of the inefficiency of early mark and sweep algorithms.
)
$(LI Modern garbage collectors do heap compaction. Heap compaction
tends to reduce the number of pages actively referenced by a program,
which means that memory accesses are more likely to be cache hits
and less swapping.
)
$(LI Garbage collected programs do not suffer from gradual deterioration
due to an accumulation of memory leaks.
)
)
)
$(LI Garbage collectors reclaim unused memory, therefore they do not suffer
from "memory leaks" which can cause long running applications to gradually
consume more and more memory until they bring down the system. GC programs
have longer term stability.
)
$(LI Garbage collected programs have fewer hard-to-find pointer bugs. This
is because there are no dangling references to freed memory. There is no
code to explicitly manage memory, hence no bugs in such code.
)
$(LI Garbage collected programs are faster to develop and debug, because
there's no need for developing, debugging, testing, or maintaining the
explicit deallocation code.
)
$(LI Garbage collected programs can be significantly smaller, because
there is no code to manage deallocation, and there is no need for exception
handlers to deallocate memory.
)
)
$(P Garbage collection is not a panacea. There are some downsides:
)
$(UL
$(LI It is not predictable when a collection gets run, so the program
can arbitrarily pause.
)
$(LI The time it takes for a collection to run is not bounded. While in
practice it is very quick, this cannot be guaranteed.
)
$(LI All threads other than the collector thread must be halted while
the collection is in progress.
)
$(LI Garbage collectors can keep around some memory that an explicit
deallocator
would not. In practice, this is not much of an issue since explicit
deallocators usually have memory leaks causing them to eventually use
far more memory, and because explicit deallocators do not normally
return deallocated memory to the operating system anyway, instead just
returning it to its own internal pool.
)
$(LI Garbage collection should be implemented as a basic operating
system
kernel service. But since they are not, garbage collecting programs must
carry around with them the garbage collection implementation. While this
can be a shared DLL, it is still there.
)
)
$(P These constraints are addressed by techniques outlined
in $(DPLLINK memory.html, Memory Management).
)
<h2>How Garbage Collection Works</h2>
$(P The GC works by:)
$(OL
$(LI Looking for all the pointer $(SINGLEQUOTE roots) into GC allocated memory.)
$(LI Recursively scanning all allocated memory pointed to by
roots looking for more pointers into GC allocated memory.)
$(LI Freeing all GC allocated memory that has no active pointers
to it.)
$(LI Possibly compacting the remaining used memory by copying the
allocated objects (called a copying collector).)
)
<h2>Interfacing Garbage Collected Objects With Foreign Code</h2>
$(P The garbage collector looks for roots in:)
$(OL
$(LI its static data segment)
$(LI the stacks and register contents of each thread)
$(LI any roots added by std.gc.addRoot() or std.gc.addRange())
)
$(P If the only root of an object
is held outside of this, then the collecter will miss it and free the
memory.
)
$(P To avoid this from happening,)
$(UL
$(LI Maintain a root to the object in an area the collector does scan
for roots.)
$(LI Add a root to the object using std.gc.addRoot() or std.gc.addRange().)
$(LI Reallocate and copy the object using the foreign code's storage
allocator
or using the C runtime library's malloc/free.
)
)
<h2>Pointers and the Garbage Collector</h2>
$(P Pointers in D can be broadly divided into two categories: those that
point to garbage collected memory, and those that do not. Examples
of the latter are pointers created by calls to C's malloc(), pointers
received from C library routines, pointers to static data,
pointers to objects on the stack, etc. For those pointers, anything
that is legal in C can be done with them.
)
$(P For garbage collected pointers and references, however, there are
some
restrictions. These restrictions are minor, but they are intended
to enable the maximum flexibility in garbage collector design.
)
$(P Undefined behavior:)
$(UL
$(LI Do not xor pointers with other values, like the
xor pointer linked list trick used in C.
)
$(LI Do not use the xor trick to swap two pointer values.
)
$(LI Do not store pointers into non-pointer variables using casts and
other tricks.
------
void* p;
...
int x = cast(int)p; // error: undefined behavior
------
The garbage collector does not scan non-pointer types for roots.
)
$(LI Do not take advantage of alignment of pointers to store bit flags
in the low order bits:
------
p = cast(void*)(cast(int)p | 1); // error: undefined behavior
------
)
$(LI Do not store into pointers values that may point into the
garbage collected heap:
------
p = cast(void*)12345678; // error: undefined behavior
------
A copying garbage collector may change this value.
)
$(LI Do not store magic values into pointers, other than $(D null).
)
$(LI Do not write pointer values out to disk and read them back in
again.
)
$(LI Do not use pointer values to compute a hash function. A copying
garbage collector can arbitrarily move objects around in memory,
thus invalidating
the computed hash value.
)
$(LI Do not depend on the ordering of pointers:
------
if (p1 < p2) // error: undefined behavior
...
------
since, again, the garbage collector can move objects around in
memory.
)
$(LI Do not add or subtract an offset to a pointer such that the result
points outside of the bounds of the garbage collected object originally
allocated.
------
char* p = new char[10];
char* q = p + 6; // ok
q = p + 11; // error: undefined behavior
q = p - 1; // error: undefined behavior
------
)
$(LI Do not misalign pointers if those pointers may
point into the gc heap, such as:
------
align (1) struct Foo {
byte b;
char* p; // misaligned pointer
}
------
Misaligned pointers may be used if the underlying hardware
supports them $(B and) the pointer is never used to point
into the gc heap.
)
$(LI Do not use byte-by-byte memory copies to copy pointer values.
This may result in intermediate conditions where there is
not a valid pointer, and if the gc pauses the thread in such a
condition, it can corrupt memory.
Most implementations of $(D memcpy()) will work since the
internal implementation of it does the copy in aligned chunks
greater than or equal to a pointer size, but since this kind of
implementation is not guaranteed by the C standard, use
$(D memcpy()) only with extreme caution.
)
$(LI Do not have pointers in a struct instance that point back
to the same instance. The trouble with this is if the instance
gets moved in memory, the pointer will point back to where it
came from, with likely disastrous results.
)
)
$(P Things that are reliable and can be done:)
$(UL
$(LI Use a union to share storage with a pointer:
------
union U { void* ptr; int value }
------
)
$(LI A pointer to the start of a garbage collected object need not
be maintained if a pointer to the interior of the object exists.
------
char[] p = new char[10];
char[] q = p[3..6];
// q is enough to hold on to the object, don't need to keep
// p as well.
------
)
)
$(P One can avoid using pointers anyway for most tasks. D provides
features
rendering most explicit pointer uses obsolete, such as reference
objects,
dynamic arrays, and garbage collection. Pointers
are provided in order to interface successfully with C APIs and for
some low level work.
)
<h2>Working with the Garbage Collector</h2>
$(P Garbage collection doesn't solve every memory deallocation problem.
For
example, if a root to a large data structure is kept, the garbage
collector cannot reclaim it, even if it is never referred to again. To
eliminate this problem, it is good practice to set a reference or
pointer to an object to null when no longer needed.
)
$(P This advice applies only to static references or references embedded
inside other objects. There is not much point for such stored on the
stack to be nulled, since the collector doesn't scan for roots past the
top of the stack, and because new stack frames are initialized anyway.
)
<h2>Object Pinning and a Moving Garbage Collector</h2>
$(P Although D does not currently use a moving garbage collector, by following
the rules listed above one can be implemented. No special action is required
to pin objects. A moving collector will only move objects for which there
are no ambiguous references, and for which it can update those references.
All other objects will be automatically pinned.
)
<h2>D Operations That Involve the Garbage Collector</h2>
$(P Some sections of code may need to avoid using the garbage collector.
The following constructs may allocate memory using the garbage collector:
)
$(UL
$(LI $(GLINK2 expression, NewExpression))
$(LI Array appending)
$(LI Array concatenation)
$(LI Array literals (except when used to initialize static data))
$(LI Associative array literals)
$(LI Any insertion, removal, or lookups in an associative array)
$(LI Extracting keys or values from an associative array)
$(V2
$(LI Taking the address of (i.e. making a delegate) a nested function that
accesses variables in an outer scope)
$(LI A function literal that access variables in an outer scope)
)
$(LI An $(GLINK2 expression, AssertExpression) that fails its condition)
)
<h2>References</h2>
$(UL
$(LI $(LINK2 http://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29, Wikipedia))
$(LI $(LINK2 http://www.iecc.com/gclist/GC-faq.html, GC FAQ))
$(LI $(LINK2 ftp://ftp.cs.utexas.edu/pub/garbage/gcsurvey.ps, Uniprocessor Garbage Collector Techniques))
$(LI $(AMAZONLINK 0471941484, Garbage Collection : Algorithms for Automatic Dynamic Memory Management))
)
)
Macros:
TITLE=Garbage Collection
WIKI=Garbage
CATEGORY_SPEC=$0
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