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<TITLE>Debugging Garbage Collector Related Problems</title> | |
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<H1>Debugging Garbage Collector Related Problems</h1> | |
This page contains some hints on | |
debugging issues specific to | |
the Boehm-Demers-Weiser conservative garbage collector. | |
It applies both to debugging issues in client code that manifest themselves | |
as collector misbehavior, and to debugging the collector itself. | |
<P> | |
If you suspect a bug in the collector itself, it is strongly recommended | |
that you try the latest collector release, even if it is labelled as "alpha", | |
before proceeding. | |
<H2>Bus Errors and Segmentation Violations</h2> | |
<P> | |
If the fault occurred in GC_find_limit, or with incremental collection enabled, | |
this is probably normal. The collector installs handlers to take care of | |
these. You will not see these unless you are using a debugger. | |
Your debugger <I>should</i> allow you to continue. | |
It's often preferable to tell the debugger to ignore SIGBUS and SIGSEGV | |
("<TT>handle SIGSEGV SIGBUS nostop noprint</tt>" in gdb, | |
"<TT>ignore SIGSEGV SIGBUS</tt>" in most versions of dbx) | |
and set a breakpoint in <TT>abort</tt>. | |
The collector will call abort if the signal had another cause, | |
and there was not other handler previously installed. | |
<P> | |
We recommend debugging without incremental collection if possible. | |
(This applies directly to UNIX systems. | |
Debugging with incremental collection under win32 is worse. See README.win32.) | |
<P> | |
If the application generates an unhandled SIGSEGV or equivalent, it may | |
often be easiest to set the environment variable GC_LOOP_ON_ABORT. On many | |
platforms, this will cause the collector to loop in a handler when the | |
SIGSEGV is encountered (or when the collector aborts for some other reason), | |
and a debugger can then be attached to the looping | |
process. This sidesteps common operating system problems related | |
to incomplete core files for multithreaded applications, etc. | |
<H2>Other Signals</h2> | |
On most platforms, the multithreaded version of the collector needs one or | |
two other signals for internal use by the collector in stopping threads. | |
It is normally wise to tell the debugger to ignore these. On Linux, | |
the collector currently uses SIGPWR and SIGXCPU by default. | |
<H2>Warning Messages About Needing to Allocate Blacklisted Blocks</h2> | |
The garbage collector generates warning messages of the form | |
<PRE> | |
Needed to allocate blacklisted block at 0x... | |
</pre> | |
or | |
<PRE> | |
Repeated allocation of very large block ... | |
</pre> | |
when it needs to allocate a block at a location that it knows to be | |
referenced by a false pointer. These false pointers can be either permanent | |
(<I>e.g.</i> a static integer variable that never changes) or temporary. | |
In the latter case, the warning is largely spurious, and the block will | |
eventually be reclaimed normally. | |
In the former case, the program will still run correctly, but the block | |
will never be reclaimed. Unless the block is intended to be | |
permanent, the warning indicates a memory leak. | |
<OL> | |
<LI>Ignore these warnings while you are using GC_DEBUG. Some of the routines | |
mentioned below don't have debugging equivalents. (Alternatively, write | |
the missing routines and send them to me.) | |
<LI>Replace allocator calls that request large blocks with calls to | |
<TT>GC_malloc_ignore_off_page</tt> or | |
<TT>GC_malloc_atomic_ignore_off_page</tt>. You may want to set a | |
breakpoint in <TT>GC_default_warn_proc</tt> to help you identify such calls. | |
Make sure that a pointer to somewhere near the beginning of the resulting block | |
is maintained in a (preferably volatile) variable as long as | |
the block is needed. | |
<LI> | |
If the large blocks are allocated with realloc, we suggest instead allocating | |
them with something like the following. Note that the realloc size increment | |
should be fairly large (e.g. a factor of 3/2) for this to exhibit reasonable | |
performance. But we all know we should do that anyway. | |
<PRE> | |
void * big_realloc(void *p, size_t new_size) | |
{ | |
size_t old_size = GC_size(p); | |
void * result; | |
if (new_size <= 10000) return(GC_realloc(p, new_size)); | |
if (new_size <= old_size) return(p); | |
result = GC_malloc_ignore_off_page(new_size); | |
if (result == 0) return(0); | |
memcpy(result,p,old_size); | |
GC_free(p); | |
return(result); | |
} | |
</pre> | |
<LI> In the unlikely case that even relatively small object | |
(<20KB) allocations are triggering these warnings, then your address | |
space contains lots of "bogus pointers", i.e. values that appear to | |
be pointers but aren't. Usually this can be solved by using GC_malloc_atomic | |
or the routines in gc_typed.h to allocate large pointer-free regions of bitmaps, etc. Sometimes the problem can be solved with trivial changes of encoding | |
in certain values. It is possible, to identify the source of the bogus | |
pointers by building the collector with <TT>-DPRINT_BLACK_LIST</tt>, | |
which will cause it to print the "bogus pointers", along with their location. | |
<LI> If you get only a fixed number of these warnings, you are probably only | |
introducing a bounded leak by ignoring them. If the data structures being | |
allocated are intended to be permanent, then it is also safe to ignore them. | |
The warnings can be turned off by calling GC_set_warn_proc with a procedure | |
that ignores these warnings (e.g. by doing absolutely nothing). | |
</ol> | |
<H2>The Collector References a Bad Address in <TT>GC_malloc</tt></h2> | |
This typically happens while the collector is trying to remove an entry from | |
its free list, and the free list pointer is bad because the free list link | |
in the last allocated object was bad. | |
<P> | |
With > 99% probability, you wrote past the end of an allocated object. | |
Try setting <TT>GC_DEBUG</tt> before including <TT>gc.h</tt> and | |
allocating with <TT>GC_MALLOC</tt>. This will try to detect such | |
overwrite errors. | |
<H2>Unexpectedly Large Heap</h2> | |
Unexpected heap growth can be due to one of the following: | |
<OL> | |
<LI> Data structures that are being unintentionally retained. This | |
is commonly caused by data structures that are no longer being used, | |
but were not cleared, or by caches growing without bounds. | |
<LI> Pointer misidentification. The garbage collector is interpreting | |
integers or other data as pointers and retaining the "referenced" | |
objects. A common symptom is that GC_dump() shows much of the heap | |
as black-listed. | |
<LI> Heap fragmentation. This should never result in unbounded growth, | |
but it may account for larger heaps. This is most commonly caused | |
by allocation of large objects. On some platforms it can be reduced | |
by building with -DUSE_MUNMAP, which will cause the collector to unmap | |
memory corresponding to pages that have not been recently used. | |
<LI> Per object overhead. This is usually a relatively minor effect, but | |
it may be worth considering. If the collector recognizes interior | |
pointers, object sizes are increased, so that one-past-the-end pointers | |
are correctly recognized. The collector can be configured not to do this | |
(<TT>-DDONT_ADD_BYTE_AT_END</tt>). | |
<P> | |
The collector rounds up object sizes so the result fits well into the | |
chunk size (<TT>HBLKSIZE</tt>, normally 4K on 32 bit machines, 8K | |
on 64 bit machines) used by the collector. Thus it may be worth avoiding | |
objects of size 2K + 1 (or 2K if a byte is being added at the end.) | |
</ol> | |
The last two cases can often be identified by looking at the output | |
of a call to <TT>GC_dump()</tt>. Among other things, it will print the | |
list of free heap blocks, and a very brief description of all chunks in | |
the heap, the object sizes they correspond to, and how many live objects | |
were found in the chunk at the last collection. | |
<P> | |
Growing data structures can usually be identified by | |
<OL> | |
<LI> Building the collector with <TT>-DKEEP_BACK_PTRS</tt>, | |
<LI> Preferably using debugging allocation (defining <TT>GC_DEBUG</tt> | |
before including <TT>gc.h</tt> and allocating with <TT>GC_MALLOC</tt>), | |
so that objects will be identified by their allocation site, | |
<LI> Running the application long enough so | |
that most of the heap is composed of "leaked" memory, and | |
<LI> Then calling <TT>GC_generate_random_backtrace()</tt> from backptr.h | |
a few times to determine why some randomly sampled objects in the heap are | |
being retained. | |
</ol> | |
<P> | |
The same technique can often be used to identify problems with false | |
pointers, by noting whether the reference chains printed by | |
<TT>GC_generate_random_backtrace()</tt> involve any misidentified pointers. | |
An alternate technique is to build the collector with | |
<TT>-DPRINT_BLACK_LIST</tt> which will cause it to report values that | |
are almost, but not quite, look like heap pointers. It is very likely that | |
actual false pointers will come from similar sources. | |
<P> | |
In the unlikely case that false pointers are an issue, it can usually | |
be resolved using one or more of the following techniques: | |
<OL> | |
<LI> Use <TT>GC_malloc_atomic</tt> for objects containing no pointers. | |
This is especially important for large arrays containing compressed data, | |
pseudo-random numbers, and the like. It is also likely to improve GC | |
performance, perhaps drastically so if the application is paging. | |
<LI> If you allocate large objects containing only | |
one or two pointers at the beginning, either try the typed allocation | |
primitives is <TT>gc_typed.h</tt>, or separate out the pointerfree component. | |
<LI> Consider using <TT>GC_malloc_ignore_off_page()</tt> | |
to allocate large objects. (See <TT>gc.h</tt> and above for details. | |
Large means > 100K in most environments.) | |
<LI> If your heap size is larger than 100MB or so, build the collector with | |
-DLARGE_CONFIG. This allows the collector to keep more precise black-list | |
information. | |
<LI> If you are using heaps close to, or larger than, a gigabyte on a 32-bit | |
machine, you may want to consider moving to a platform with 64-bit pointers. | |
This is very likely to resolve any false pointer issues. | |
</ol> | |
<H2>Prematurely Reclaimed Objects</h2> | |
The usual symptom of this is a segmentation fault, or an obviously overwritten | |
value in a heap object. This should, of course, be impossible. In practice, | |
it may happen for reasons like the following: | |
<OL> | |
<LI> The collector did not intercept the creation of threads correctly in | |
a multithreaded application, <I>e.g.</i> because the client called | |
<TT>pthread_create</tt> without including <TT>gc.h</tt>, which redefines it. | |
<LI> The last pointer to an object in the garbage collected heap was stored | |
somewhere were the collector couldn't see it, <I>e.g.</i> in an | |
object allocated with system <TT>malloc</tt>, in certain types of | |
<TT>mmap</tt>ed files, | |
or in some data structure visible only to the OS. (On some platforms, | |
thread-local storage is one of these.) | |
<LI> The last pointer to an object was somehow disguised, <I>e.g.</i> by | |
XORing it with another pointer. | |
<LI> Incorrect use of <TT>GC_malloc_atomic</tt> or typed allocation. | |
<LI> An incorrect <TT>GC_free</tt> call. | |
<LI> The client program overwrote an internal garbage collector data structure. | |
<LI> A garbage collector bug. | |
<LI> (Empirically less likely than any of the above.) A compiler optimization | |
that disguised the last pointer. | |
</ol> | |
The following relatively simple techniques should be tried first to narrow | |
down the problem: | |
<OL> | |
<LI> If you are using the incremental collector try turning it off for | |
debugging. | |
<LI> If you are using shared libraries, try linking statically. If that works, | |
ensure that DYNAMIC_LOADING is defined on your platform. | |
<LI> Try to reproduce the problem with fully debuggable unoptimized code. | |
This will eliminate the last possibility, as well as making debugging easier. | |
<LI> Try replacing any suspect typed allocation and <TT>GC_malloc_atomic</tt> | |
calls with calls to <TT>GC_malloc</tt>. | |
<LI> Try removing any GC_free calls (<I>e.g.</i> with a suitable | |
<TT>#define</tt>). | |
<LI> Rebuild the collector with <TT>-DGC_ASSERTIONS</tt>. | |
<LI> If the following works on your platform (i.e. if gctest still works | |
if you do this), try building the collector with | |
<TT>-DREDIRECT_MALLOC=GC_malloc_uncollectable</tt>. This will cause | |
the collector to scan memory allocated with malloc. | |
</ol> | |
If all else fails, you will have to attack this with a debugger. | |
Suggested steps: | |
<OL> | |
<LI> Call <TT>GC_dump()</tt> from the debugger around the time of the failure. Verify | |
that the collectors idea of the root set (i.e. static data regions which | |
it should scan for pointers) looks plausible. If not, i.e. if it doesn't | |
include some static variables, report this as | |
a collector bug. Be sure to describe your platform precisely, since this sort | |
of problem is nearly always very platform dependent. | |
<LI> Especially if the failure is not deterministic, try to isolate it to | |
a relatively small test case. | |
<LI> Set a break point in <TT>GC_finish_collection</tt>. This is a good | |
point to examine what has been marked, i.e. found reachable, by the | |
collector. | |
<LI> If the failure is deterministic, run the process | |
up to the last collection before the failure. | |
Note that the variable <TT>GC_gc_no</tt> counts collections and can be used | |
to set a conditional breakpoint in the right one. It is incremented just | |
before the call to GC_finish_collection. | |
If object <TT>p</tt> was prematurely recycled, it may be helpful to | |
look at <TT>*GC_find_header(p)</tt> at the failure point. | |
The <TT>hb_last_reclaimed</tt> field will identify the collection number | |
during which its block was last swept. | |
<LI> Verify that the offending object still has its correct contents at | |
this point. | |
Then call <TT>GC_is_marked(p)</tt> from the debugger to verify that the | |
object has not been marked, and is about to be reclaimed. Note that | |
<TT>GC_is_marked(p)</tt> expects the real address of an object (the | |
address of the debug header if there is one), and thus it may | |
be more appropriate to call <TT>GC_is_marked(GC_base(p))</tt> | |
instead. | |
<LI> Determine a path from a root, i.e. static variable, stack, or | |
register variable, | |
to the reclaimed object. Call <TT>GC_is_marked(q)</tt> for each object | |
<TT>q</tt> along the path, trying to locate the first unmarked object, say | |
<TT>r</tt>. | |
<LI> If <TT>r</tt> is pointed to by a static root, | |
verify that the location | |
pointing to it is part of the root set printed by <TT>GC_dump()</tt>. If it | |
is on the stack in the main (or only) thread, verify that | |
<TT>GC_stackbottom</tt> is set correctly to the base of the stack. If it is | |
in another thread stack, check the collector's thread data structure | |
(<TT>GC_thread[]</tt> on several platforms) to make sure that stack bounds | |
are set correctly. | |
<LI> If <TT>r</tt> is pointed to by heap object <TT>s</tt>, check that the | |
collector's layout description for <TT>s</tt> is such that the pointer field | |
will be scanned. Call <TT>*GC_find_header(s)</tt> to look at the descriptor | |
for the heap chunk. The <TT>hb_descr</tt> field specifies the layout | |
of objects in that chunk. See gc_mark.h for the meaning of the descriptor. | |
(If it's low order 2 bits are zero, then it is just the length of the | |
object prefix to be scanned. This form is always used for objects allocated | |
with <TT>GC_malloc</tt> or <TT>GC_malloc_atomic</tt>.) | |
<LI> If the failure is not deterministic, you may still be able to apply some | |
of the above technique at the point of failure. But remember that objects | |
allocated since the last collection will not have been marked, even if the | |
collector is functioning properly. On some platforms, the collector | |
can be configured to save call chains in objects for debugging. | |
Enabling this feature will also cause it to save the call stack at the | |
point of the last GC in GC_arrays._last_stack. | |
<LI> When looking at GC internal data structures remember that a number | |
of <TT>GC_</tt><I>xxx</i> variables are really macro defined to | |
<TT>GC_arrays._</tt><I>xxx</i>, so that | |
the collector can avoid scanning them. | |
</ol> | |
</body> | |
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