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| 2 | <HEAD>
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| 3 | <TITLE>Debugging Garbage Collector Related Problems</title>
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| 4 | </head>
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| 5 | <BODY>
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| 6 | <H1>Debugging Garbage Collector Related Problems</h1>
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| 7 | This page contains some hints on
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| 8 | debugging issues specific to
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| 9 | the Boehm-Demers-Weiser conservative garbage collector.
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| 10 | It applies both to debugging issues in client code that manifest themselves
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| 11 | as collector misbehavior, and to debugging the collector itself.
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| 12 | <P>
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| 13 | If you suspect a bug in the collector itself, it is strongly recommended
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| 14 | that you try the latest collector release, even if it is labelled as "alpha",
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| 15 | before proceeding.
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| 16 | <H2>Bus Errors and Segmentation Violations</h2>
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| 17 | <P>
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| 18 | If the fault occurred in GC_find_limit, or with incremental collection enabled,
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| 19 | this is probably normal. The collector installs handlers to take care of
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| 20 | these. You will not see these unless you are using a debugger.
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| 21 | Your debugger <I>should</i> allow you to continue.
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| 22 | It's often preferable to tell the debugger to ignore SIGBUS and SIGSEGV
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| 23 | ("<TT>handle SIGSEGV SIGBUS nostop noprint</tt>" in gdb,
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| 24 | "<TT>ignore SIGSEGV SIGBUS</tt>" in most versions of dbx)
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| 25 | and set a breakpoint in <TT>abort</tt>.
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| 26 | The collector will call abort if the signal had another cause,
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| 27 | and there was not other handler previously installed.
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| 28 | <P>
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| 29 | We recommend debugging without incremental collection if possible.
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| 30 | (This applies directly to UNIX systems.
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| 31 | Debugging with incremental collection under win32 is worse. See README.win32.)
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| 32 | <P>
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| 33 | If the application generates an unhandled SIGSEGV or equivalent, it may
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| 34 | often be easiest to set the environment variable GC_LOOP_ON_ABORT. On many
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| 35 | platforms, this will cause the collector to loop in a handler when the
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| 36 | SIGSEGV is encountered (or when the collector aborts for some other reason),
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| 37 | and a debugger can then be attached to the looping
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| 38 | process. This sidesteps common operating system problems related
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| 39 | to incomplete core files for multithreaded applications, etc.
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| 40 | <H2>Other Signals</h2>
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| 41 | On most platforms, the multithreaded version of the collector needs one or
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| 42 | two other signals for internal use by the collector in stopping threads.
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| 43 | It is normally wise to tell the debugger to ignore these. On Linux,
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| 44 | the collector currently uses SIGPWR and SIGXCPU by default.
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| 45 | <H2>Warning Messages About Needing to Allocate Blacklisted Blocks</h2>
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| 46 | The garbage collector generates warning messages of the form
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| 47 | <PRE>
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| 48 | Needed to allocate blacklisted block at 0x...
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| 49 | </pre>
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| 50 | when it needs to allocate a block at a location that it knows to be
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| 51 | referenced by a false pointer. These false pointers can be either permanent
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| 52 | (<I>e.g.</i> a static integer variable that never changes) or temporary.
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| 53 | In the latter case, the warning is largely spurious, and the block will
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| 54 | eventually be reclaimed normally.
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| 55 | In the former case, the program will still run correctly, but the block
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| 56 | will never be reclaimed. Unless the block is intended to be
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| 57 | permanent, the warning indicates a memory leak.
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| 58 | <OL>
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| 59 | <LI>Ignore these warnings while you are using GC_DEBUG. Some of the routines
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| 60 | mentioned below don't have debugging equivalents. (Alternatively, write
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| 61 | the missing routines and send them to me.)
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| 62 | <LI>Replace allocator calls that request large blocks with calls to
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| 63 | <TT>GC_malloc_ignore_off_page</tt> or
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| 64 | <TT>GC_malloc_atomic_ignore_off_page</tt>. You may want to set a
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| 65 | breakpoint in <TT>GC_default_warn_proc</tt> to help you identify such calls.
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| 66 | Make sure that a pointer to somewhere near the beginning of the resulting block
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| 67 | is maintained in a (preferably volatile) variable as long as
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| 68 | the block is needed.
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| 69 | <LI>
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| 70 | If the large blocks are allocated with realloc, we suggest instead allocating
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| 71 | them with something like the following. Note that the realloc size increment
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| 72 | should be fairly large (e.g. a factor of 3/2) for this to exhibit reasonable
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| 73 | performance. But we all know we should do that anyway.
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| 74 | <PRE>
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| 75 | void * big_realloc(void *p, size_t new_size)
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| 76 | {
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| 77 | size_t old_size = GC_size(p);
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| 78 | void * result;
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| 79 |
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| 80 | if (new_size <= 10000) return(GC_realloc(p, new_size));
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| 81 | if (new_size <= old_size) return(p);
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| 82 | result = GC_malloc_ignore_off_page(new_size);
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| 83 | if (result == 0) return(0);
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| 84 | memcpy(result,p,old_size);
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| 85 | GC_free(p);
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| 86 | return(result);
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| 87 | }
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| 88 | </pre>
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| 89 |
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| 90 | <LI> In the unlikely case that even relatively small object
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| 91 | (<20KB) allocations are triggering these warnings, then your address
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| 92 | space contains lots of "bogus pointers", i.e. values that appear to
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| 93 | be pointers but aren't. Usually this can be solved by using GC_malloc_atomic
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| 94 | 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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| 95 | in certain values. It is possible, to identify the source of the bogus
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| 96 | pointers by building the collector with <TT>-DPRINT_BLACK_LIST</tt>,
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| 97 | which will cause it to print the "bogus pointers", along with their location.
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| 98 |
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| 99 | <LI> If you get only a fixed number of these warnings, you are probably only
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| 100 | introducing a bounded leak by ignoring them. If the data structures being
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| 101 | allocated are intended to be permanent, then it is also safe to ignore them.
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| 102 | The warnings can be turned off by calling GC_set_warn_proc with a procedure
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| 103 | that ignores these warnings (e.g. by doing absolutely nothing).
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| 104 | </ol>
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| 105 |
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| 106 | <H2>The Collector References a Bad Address in <TT>GC_malloc</tt></h2>
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| 107 |
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| 108 | This typically happens while the collector is trying to remove an entry from
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| 109 | its free list, and the free list pointer is bad because the free list link
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| 110 | in the last allocated object was bad.
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| 111 | <P>
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| 112 | With > 99% probability, you wrote past the end of an allocated object.
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| 113 | Try setting <TT>GC_DEBUG</tt> before including <TT>gc.h</tt> and
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| 114 | allocating with <TT>GC_MALLOC</tt>. This will try to detect such
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| 115 | overwrite errors.
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| 116 |
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| 117 | <H2>Unexpectedly Large Heap</h2>
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| 118 |
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| 119 | Unexpected heap growth can be due to one of the following:
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| 120 | <OL>
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| 121 | <LI> Data structures that are being unintentionally retained. This
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| 122 | is commonly caused by data structures that are no longer being used,
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| 123 | but were not cleared, or by caches growing without bounds.
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| 124 | <LI> Pointer misidentification. The garbage collector is interpreting
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| 125 | integers or other data as pointers and retaining the "referenced"
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| 126 | objects.
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| 127 | <LI> Heap fragmentation. This should never result in unbounded growth,
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| 128 | but it may account for larger heaps. This is most commonly caused
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| 129 | by allocation of large objects. On some platforms it can be reduced
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| 130 | by building with -DUSE_MUNMAP, which will cause the collector to unmap
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| 131 | memory corresponding to pages that have not been recently used.
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| 132 | <LI> Per object overhead. This is usually a relatively minor effect, but
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| 133 | it may be worth considering. If the collector recognizes interior
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| 134 | pointers, object sizes are increased, so that one-past-the-end pointers
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| 135 | are correctly recognized. The collector can be configured not to do this
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| 136 | (<TT>-DDONT_ADD_BYTE_AT_END</tt>).
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| 137 | <P>
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| 138 | The collector rounds up object sizes so the result fits well into the
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| 139 | chunk size (<TT>HBLKSIZE</tt>, normally 4K on 32 bit machines, 8K
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| 140 | on 64 bit machines) used by the collector. Thus it may be worth avoiding
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| 141 | objects of size 2K + 1 (or 2K if a byte is being added at the end.)
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| 142 | </ol>
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| 143 | The last two cases can often be identified by looking at the output
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| 144 | of a call to <TT>GC_dump()</tt>. Among other things, it will print the
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| 145 | list of free heap blocks, and a very brief description of all chunks in
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| 146 | the heap, the object sizes they correspond to, and how many live objects
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| 147 | were found in the chunk at the last collection.
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| 148 | <P>
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| 149 | Growing data structures can usually be identified by
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| 150 | <OL>
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| 151 | <LI> Building the collector with <TT>-DKEEP_BACK_PTRS</tt>,
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| 152 | <LI> Preferably using debugging allocation (defining <TT>GC_DEBUG</tt>
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| 153 | before including <TT>gc.h</tt> and allocating with <TT>GC_MALLOC</tt>),
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| 154 | so that objects will be identified by their allocation site,
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| 155 | <LI> Running the application long enough so
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| 156 | that most of the heap is composed of "leaked" memory, and
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| 157 | <LI> Then calling <TT>GC_generate_random_backtrace()</tt> from backptr.h
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| 158 | a few times to determine why some randomly sampled objects in the heap are
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| 159 | being retained.
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| 160 | </ol>
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| 161 | <P>
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| 162 | The same technique can often be used to identify problems with false
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| 163 | pointers, by noting whether the reference chains printed by
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| 164 | <TT>GC_generate_random_backtrace()</tt> involve any misidentified pointers.
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| 165 | An alternate technique is to build the collector with
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| 166 | <TT>-DPRINT_BLACK_LIST</tt> which will cause it to report values that
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| 167 | are almost, but not quite, look like heap pointers. It is very likely that
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| 168 | actual false pointers will come from similar sources.
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| 169 | <P>
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| 170 | In the unlikely case that false pointers are an issue, it can usually
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| 171 | be resolved using one or more of the following techniques:
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| 172 | <OL>
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| 173 | <LI> Use <TT>GC_malloc_atomic</tt> for objects containing no pointers.
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| 174 | This is especially important for large arrays containing compressed data,
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| 175 | pseudo-random numbers, and the like. It is also likely to improve GC
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| 176 | performance, perhaps drastically so if the application is paging.
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| 177 | <LI> If you allocate large objects containing only
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| 178 | one or two pointers at the beginning, either try the typed allocation
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| 179 | primitives is <TT>gc_typed.h</tt>, or separate out the pointerfree component.
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| 180 | <LI> Consider using <TT>GC_malloc_ignore_off_page()</tt>
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| 181 | to allocate large objects. (See <TT>gc.h</tt> and above for details.
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| 182 | Large means > 100K in most environments.)
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| 183 | </ol>
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| 184 | <H2>Prematurely Reclaimed Objects</h2>
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| 185 | The usual symptom of this is a segmentation fault, or an obviously overwritten
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| 186 | value in a heap object. This should, of course, be impossible. In practice,
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| 187 | it may happen for reasons like the following:
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| 188 | <OL>
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| 189 | <LI> The collector did not intercept the creation of threads correctly in
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| 190 | a multithreaded application, <I>e.g.</i> because the client called
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| 191 | <TT>pthread_create</tt> without including <TT>gc.h</tt>, which redefines it.
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| 192 | <LI> The last pointer to an object in the garbage collected heap was stored
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| 193 | somewhere were the collector couldn't see it, <I>e.g.</i> in an
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| 194 | object allocated with system <TT>malloc</tt>, in certain types of
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| 195 | <TT>mmap</tt>ed files,
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| 196 | or in some data structure visible only to the OS. (On some platforms,
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| 197 | thread-local storage is one of these.)
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| 198 | <LI> The last pointer to an object was somehow disguised, <I>e.g.</i> by
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| 199 | XORing it with another pointer.
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| 200 | <LI> Incorrect use of <TT>GC_malloc_atomic</tt> or typed allocation.
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| 201 | <LI> An incorrect <TT>GC_free</tt> call.
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| 202 | <LI> The client program overwrote an internal garbage collector data structure.
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| 203 | <LI> A garbage collector bug.
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| 204 | <LI> (Empirically less likely than any of the above.) A compiler optimization
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| 205 | that disguised the last pointer.
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| 206 | </ol>
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| 207 | The following relatively simple techniques should be tried first to narrow
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| 208 | down the problem:
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| 209 | <OL>
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| 210 | <LI> If you are using the incremental collector try turning it off for
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| 211 | debugging.
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| 212 | <LI> If you are using shared libraries, try linking statically. If that works,
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| 213 | ensure that DYNAMIC_LOADING is defined on your platform.
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| 214 | <LI> Try to reproduce the problem with fully debuggable unoptimized code.
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| 215 | This will eliminate the last possibility, as well as making debugging easier.
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| 216 | <LI> Try replacing any suspect typed allocation and <TT>GC_malloc_atomic</tt>
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| 217 | calls with calls to <TT>GC_malloc</tt>.
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| 218 | <LI> Try removing any GC_free calls (<I>e.g.</i> with a suitable
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| 219 | <TT>#define</tt>).
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| 220 | <LI> Rebuild the collector with <TT>-DGC_ASSERTIONS</tt>.
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| 221 | <LI> If the following works on your platform (i.e. if gctest still works
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| 222 | if you do this), try building the collector with
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| 223 | <TT>-DREDIRECT_MALLOC=GC_malloc_uncollectable</tt>. This will cause
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| 224 | the collector to scan memory allocated with malloc.
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| 225 | </ol>
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| 226 | If all else fails, you will have to attack this with a debugger.
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| 227 | Suggested steps:
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| 228 | <OL>
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| 229 | <LI> Call <TT>GC_dump()</tt> from the debugger around the time of the failure. Verify
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| 230 | that the collectors idea of the root set (i.e. static data regions which
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| 231 | it should scan for pointers) looks plausible. If not, i.e. if it doesn't
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| 232 | include some static variables, report this as
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| 233 | a collector bug. Be sure to describe your platform precisely, since this sort
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| 234 | of problem is nearly always very platform dependent.
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| 235 | <LI> Especially if the failure is not deterministic, try to isolate it to
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| 236 | a relatively small test case.
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| 237 | <LI> Set a break point in <TT>GC_finish_collection</tt>. This is a good
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| 238 | point to examine what has been marked, i.e. found reachable, by the
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| 239 | collector.
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| 240 | <LI> If the failure is deterministic, run the process
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| 241 | up to the last collection before the failure.
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| 242 | Note that the variable <TT>GC_gc_no</tt> counts collections and can be used
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| 243 | to set a conditional breakpoint in the right one. It is incremented just
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| 244 | before the call to GC_finish_collection.
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| 245 | If object <TT>p</tt> was prematurely recycled, it may be helpful to
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| 246 | look at <TT>*GC_find_header(p)</tt> at the failure point.
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| 247 | The <TT>hb_last_reclaimed</tt> field will identify the collection number
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| 248 | during which its block was last swept.
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| 249 | <LI> Verify that the offending object still has its correct contents at
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| 250 | this point.
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| 251 | The call <TT>GC_is_marked(p)</tt> from the debugger to verify that the
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| 252 | object has not been marked, and is about to be reclaimed.
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| 253 | <LI> Determine a path from a root, i.e. static variable, stack, or
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| 254 | register variable,
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| 255 | to the reclaimed object. Call <TT>GC_is_marked(q)</tt> for each object
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| 256 | <TT>q</tt> along the path, trying to locate the first unmarked object, say
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| 257 | <TT>r</tt>.
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| 258 | <LI> If <TT>r</tt> is pointed to by a static root,
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| 259 | verify that the location
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| 260 | pointing to it is part of the root set printed by <TT>GC_dump()</tt>. If it
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| 261 | is on the stack in the main (or only) thread, verify that
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| 262 | <TT>GC_stackbottom</tt> is set correctly to the base of the stack. If it is
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| 263 | in another thread stack, check the collector's thread data structure
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| 264 | (<TT>GC_thread[]</tt> on several platforms) to make sure that stack bounds
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| 265 | are set correctly.
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| 266 | <LI> If <TT>r</tt> is pointed to by heap object <TT>s</tt>, check that the
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| 267 | collector's layout description for <TT>s</tt> is such that the pointer field
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| 268 | will be scanned. Call <TT>*GC_find_header(s)</tt> to look at the descriptor
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| 269 | for the heap chunk. The <TT>hb_descr</tt> field specifies the layout
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| 270 | of objects in that chunk. See gc_mark.h for the meaning of the descriptor.
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| 271 | (If it's low order 2 bits are zero, then it is just the length of the
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| 272 | object prefix to be scanned. This form is always used for objects allocated
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| 273 | with <TT>GC_malloc</tt> or <TT>GC_malloc_atomic</tt>.)
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| 274 | <LI> If the failure is not deterministic, you may still be able to apply some
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| 275 | of the above technique at the point of failure. But remember that objects
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| 276 | allocated since the last collection will not have been marked, even if the
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| 277 | collector is functioning properly. On some platforms, the collector
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| 278 | can be configured to save call chains in objects for debugging.
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| 279 | Enabling this feature will also cause it to save the call stack at the
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| 280 | point of the last GC in GC_arrays._last_stack.
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| 281 | <LI> When looking at GC internal data structures remember that a number
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| 282 | of <TT>GC_</tt><I>xxx</i> variables are really macro defined to
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| 283 | <TT>GC_arrays._</tt><I>xxx</i>, so that
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| 284 | the collector can avoid scanning them.
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| 285 | </ol>
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| 286 | </body>
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| 287 | </html>
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| 288 |
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| 289 |
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| 290 |
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| 291 |
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