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