boehm-gc/doc/debugging.html

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