Annotation of OpenXM_contrib2/asir2000/gc/doc/debugging.html, Revision 1.2
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2: <HEAD>
3: <TITLE>Debugging Garbage Collector Related Problems</title>
4: </head>
5: <BODY>
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: (<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 > 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 > 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.
1.2 ! noro 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.
1.1 noro 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>
291: </html>
292:
293:
294:
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