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