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diff --git a/gc/doc/debugging.html b/gc/doc/debugging.html new file mode 100644 index 0000000..a186ff5 --- /dev/null +++ b/gc/doc/debugging.html @@ -0,0 +1,289 @@ +<HTML> +<HEAD> +<TITLE>Debugging Garbage Collector Related Problems</title> +</head> +<BODY> +<H1>Debugging Garbage Collector Related Problems</h1> +This page contains some hints on +debugging issues specific to +the Boehm-Demers-Weiser conservative garbage collector. +It applies both to debugging issues in client code that manifest themselves +as collector misbehavior, and to debugging the collector itself. +<P> +If you suspect a bug in the collector itself, it is strongly recommended +that you try the latest collector release, even if it is labelled as "alpha", +before proceeding. +<H2>Bus Errors and Segmentation Violations</h2> +<P> +If the fault occurred in GC_find_limit, or with incremental collection enabled, +this is probably normal. The collector installs handlers to take care of +these. You will not see these unless you are using a debugger. +Your debugger <I>should</i> allow you to continue. +It's often preferable to tell the debugger to ignore SIGBUS and SIGSEGV +("<TT>handle SIGSEGV SIGBUS nostop noprint</tt>" in gdb, +"<TT>ignore SIGSEGV SIGBUS</tt>" in most versions of dbx) +and set a breakpoint in <TT>abort</tt>. +The collector will call abort if the signal had another cause, +and there was not other handler previously installed. +<P> +We recommend debugging without incremental collection if possible. +(This applies directly to UNIX systems. +Debugging with incremental collection under win32 is worse. See README.win32.) +<P> +If the application generates an unhandled SIGSEGV or equivalent, it may +often be easiest to set the environment variable GC_LOOP_ON_ABORT. On many +platforms, this will cause the collector to loop in a handler when the +SIGSEGV is encountered (or when the collector aborts for some other reason), +and a debugger can then be attached to the looping +process. This sidesteps common operating system problems related +to incomplete core files for multithreaded applications, etc. +<H2>Other Signals</h2> +On most platforms, the multithreaded version of the collector needs one or +two other signals for internal use by the collector in stopping threads. +It is normally wise to tell the debugger to ignore these. On Linux, +the collector currently uses SIGPWR and SIGXCPU by default. +<H2>Warning Messages About Needing to Allocate Blacklisted Blocks</h2> +The garbage collector generates warning messages of the form +<PRE> +Needed to allocate blacklisted block at 0x... +</pre> +when it needs to allocate a block at a location that it knows to be +referenced by a false pointer. These false pointers can be either permanent +(<I>e.g.</i> a static integer variable that never changes) or temporary. +In the latter case, the warning is largely spurious, and the block will +eventually be reclaimed normally. +In the former case, the program will still run correctly, but the block +will never be reclaimed. Unless the block is intended to be +permanent, the warning indicates a memory leak. +<OL> +<LI>Ignore these warnings while you are using GC_DEBUG. Some of the routines +mentioned below don't have debugging equivalents. (Alternatively, write +the missing routines and send them to me.) +<LI>Replace allocator calls that request large blocks with calls to +<TT>GC_malloc_ignore_off_page</tt> or +<TT>GC_malloc_atomic_ignore_off_page</tt>. You may want to set a +breakpoint in <TT>GC_default_warn_proc</tt> to help you identify such calls. +Make sure that a pointer to somewhere near the beginning of the resulting block +is maintained in a (preferably volatile) variable as long as +the block is needed. +<LI> +If the large blocks are allocated with realloc, we suggest instead allocating +them with something like the following. Note that the realloc size increment +should be fairly large (e.g. a factor of 3/2) for this to exhibit reasonable +performance. But we all know we should do that anyway. +<PRE> +void * big_realloc(void *p, size_t new_size) +{ + size_t old_size = GC_size(p); + void * result; + + if (new_size <= 10000) return(GC_realloc(p, new_size)); + if (new_size <= old_size) return(p); + result = GC_malloc_ignore_off_page(new_size); + if (result == 0) return(0); + memcpy(result,p,old_size); + GC_free(p); + return(result); +} +</pre> + +<LI> In the unlikely case that even relatively small object +(<20KB) allocations are triggering these warnings, then your address +space contains lots of "bogus pointers", i.e. values that appear to +be pointers but aren't. Usually this can be solved by using GC_malloc_atomic +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 +in certain values. It is possible, to identify the source of the bogus +pointers by building the collector with <TT>-DPRINT_BLACK_LIST</tt>, +which will cause it to print the "bogus pointers", along with their location. + +<LI> If you get only a fixed number of these warnings, you are probably only +introducing a bounded leak by ignoring them. If the data structures being +allocated are intended to be permanent, then it is also safe to ignore them. +The warnings can be turned off by calling GC_set_warn_proc with a procedure +that ignores these warnings (e.g. by doing absolutely nothing). +</ol> + +<H2>The Collector References a Bad Address in <TT>GC_malloc</tt></h2> + +This typically happens while the collector is trying to remove an entry from +its free list, and the free list pointer is bad because the free list link +in the last allocated object was bad. +<P> +With > 99% probability, you wrote past the end of an allocated object. +Try setting <TT>GC_DEBUG</tt> before including <TT>gc.h</tt> and +allocating with <TT>GC_MALLOC</tt>. This will try to detect such +overwrite errors. + +<H2>Unexpectedly Large Heap</h2> + +Unexpected heap growth can be due to one of the following: +<OL> +<LI> Data structures that are being unintentionally retained. This +is commonly caused by data structures that are no longer being used, +but were not cleared, or by caches growing without bounds. +<LI> Pointer misidentification. The garbage collector is interpreting +integers or other data as pointers and retaining the "referenced" +objects. +<LI> Heap fragmentation. This should never result in unbounded growth, +but it may account for larger heaps. This is most commonly caused +by allocation of large objects. On some platforms it can be reduced +by building with -DUSE_MUNMAP, which will cause the collector to unmap +memory corresponding to pages that have not been recently used. +<LI> Per object overhead. This is usually a relatively minor effect, but +it may be worth considering. If the collector recognizes interior +pointers, object sizes are increased, so that one-past-the-end pointers +are correctly recognized. The collector can be configured not to do this +(<TT>-DDONT_ADD_BYTE_AT_END</tt>). +<P> +The collector rounds up object sizes so the result fits well into the +chunk size (<TT>HBLKSIZE</tt>, normally 4K on 32 bit machines, 8K +on 64 bit machines) used by the collector. Thus it may be worth avoiding +objects of size 2K + 1 (or 2K if a byte is being added at the end.) +</ol> +The last two cases can often be identified by looking at the output +of a call to <TT>GC_dump()</tt>. Among other things, it will print the +list of free heap blocks, and a very brief description of all chunks in +the heap, the object sizes they correspond to, and how many live objects +were found in the chunk at the last collection. +<P> +Growing data structures can usually be identified by +<OL> +<LI> Building the collector with <TT>-DKEEP_BACK_PTRS</tt>, +<LI> Preferably using debugging allocation (defining <TT>GC_DEBUG</tt> +before including <TT>gc.h</tt> and allocating with <TT>GC_MALLOC</tt>), +so that objects will be identified by their allocation site, +<LI> Running the application long enough so +that most of the heap is composed of "leaked" memory, and +<LI> Then calling <TT>GC_generate_random_backtrace()</tt> from backptr.h +a few times to determine why some randomly sampled objects in the heap are +being retained. +</ol> +<P> +The same technique can often be used to identify problems with false +pointers, by noting whether the reference chains printed by +<TT>GC_generate_random_backtrace()</tt> involve any misidentified pointers. +An alternate technique is to build the collector with +<TT>-DPRINT_BLACK_LIST</tt> which will cause it to report values that +are almost, but not quite, look like heap pointers. It is very likely that +actual false pointers will come from similar sources. +<P> +In the unlikely case that false pointers are an issue, it can usually +be resolved using one or more of the following techniques: +<OL> +<LI> Use <TT>GC_malloc_atomic</tt> for objects containing no pointers. +This is especially important for large arrays containing compressed data, +pseudo-random numbers, and the like. It is also likely to improve GC +performance, perhaps drastically so if the application is paging. +<LI> If you allocate large objects containing only +one or two pointers at the beginning, either try the typed allocation +primitives is <TT>gc_typed.h</tt>, or separate out the pointerfree component. +<LI> Consider using <TT>GC_malloc_ignore_off_page()</tt> +to allocate large objects. (See <TT>gc.h</tt> and above for details. +Large means > 100K in most environments.) +</ol> +<H2>Prematurely Reclaimed Objects</h2> +The usual symptom of this is a segmentation fault, or an obviously overwritten +value in a heap object. This should, of course, be impossible. In practice, +it may happen for reasons like the following: +<OL> +<LI> The collector did not intercept the creation of threads correctly in +a multithreaded application, <I>e.g.</i> because the client called +<TT>pthread_create</tt> without including <TT>gc.h</tt>, which redefines it. +<LI> The last pointer to an object in the garbage collected heap was stored +somewhere were the collector couldn't see it, <I>e.g.</i> in an +object allocated with system <TT>malloc</tt>, in certain types of +<TT>mmap</tt>ed files, +or in some data structure visible only to the OS. (On some platforms, +thread-local storage is one of these.) +<LI> The last pointer to an object was somehow disguised, <I>e.g.</i> by +XORing it with another pointer. +<LI> Incorrect use of <TT>GC_malloc_atomic</tt> or typed allocation. +<LI> An incorrect <TT>GC_free</tt> call. +<LI> The client program overwrote an internal garbage collector data structure. +<LI> A garbage collector bug. +<LI> (Empirically less likely than any of the above.) A compiler optimization +that disguised the last pointer. +</ol> +The following relatively simple techniques should be tried first to narrow +down the problem: +<OL> +<LI> If you are using the incremental collector try turning it off for +debugging. +<LI> Try to reproduce the problem with fully debuggable unoptimized code. +This will eliminate the last possibility, as well as making debugging easier. +<LI> Try replacing any suspect typed allocation and <TT>GC_malloc_atomic</tt> +calls with calls to <TT>GC_malloc</tt>. +<LI> Try removing any GC_free calls (<I>e.g.</i> with a suitable +<TT>#define</tt>). +<LI> Rebuild the collector with <TT>-DGC_ASSERTIONS</tt>. +<LI> If the following works on your platform (i.e. if gctest still works +if you do this), try building the collector with +<TT>-DREDIRECT_MALLOC=GC_malloc_uncollectable</tt>. This will cause +the collector to scan memory allocated with malloc. +</ol> +If all else fails, you will have to attack this with a debugger. +Suggested steps: +<OL> +<LI> Call <TT>GC_dump()</tt> from the debugger around the time of the failure. Verify +that the collectors idea of the root set (i.e. static data regions which +it should scan for pointers) looks plausible. If not, i.e. if it doesn't +include some static variables, report this as +a collector bug. Be sure to describe your platform precisely, since this sort +of problem is nearly always very platform dependent. +<LI> Especially if the failure is not deterministic, try to isolate it to +a relatively small test case. +<LI> Set a break point in <TT>GC_finish_collection</tt>. This is a good +point to examine what has been marked, i.e. found reachable, by the +collector. +<LI> If the failure is deterministic, run the process +up to the last collection before the failure. +Note that the variable <TT>GC_gc_no</tt> counts collections and can be used +to set a conditional breakpoint in the right one. It is incremented just +before the call to GC_finish_collection. +If object <TT>p</tt> was prematurely recycled, it may be helpful to +look at <TT>*GC_find_header(p)</tt> at the failure point. +The <TT>hb_last_reclaimed</tt> field will identify the collection number +during which its block was last swept. +<LI> Verify that the offending object still has its correct contents at +this point. +The call <TT>GC_is_marked(p)</tt> from the debugger to verify that the +object has not been marked, and is about to be reclaimed. +<LI> Determine a path from a root, i.e. static variable, stack, or +register variable, +to the reclaimed object. Call <TT>GC_is_marked(q)</tt> for each object +<TT>q</tt> along the path, trying to locate the first unmarked object, say +<TT>r</tt>. +<LI> If <TT>r</tt> is pointed to by a static root, +verify that the location +pointing to it is part of the root set printed by <TT>GC_dump()</tt>. If it +is on the stack in the main (or only) thread, verify that +<TT>GC_stackbottom</tt> is set correctly to the base of the stack. If it is +in another thread stack, check the collector's thread data structure +(<TT>GC_thread[]</tt> on several platforms) to make sure that stack bounds +are set correctly. +<LI> If <TT>r</tt> is pointed to by heap object <TT>s</tt>, check that the +collector's layout description for <TT>s</tt> is such that the pointer field +will be scanned. Call <TT>*GC_find_header(s)</tt> to look at the descriptor +for the heap chunk. The <TT>hb_descr</tt> field specifies the layout +of objects in that chunk. See gc_mark.h for the meaning of the descriptor. +(If it's low order 2 bits are zero, then it is just the length of the +object prefix to be scanned. This form is always used for objects allocated +with <TT>GC_malloc</tt> or <TT>GC_malloc_atomic</tt>.) +<LI> If the failure is not deterministic, you may still be able to apply some +of the above technique at the point of failure. But remember that objects +allocated since the last collection will not have been marked, even if the +collector is functioning properly. On some platforms, the collector +can be configured to save call chains in objects for debugging. +Enabling this feature will also cause it to save the call stack at the +point of the last GC in GC_arrays._last_stack. +<LI> When looking at GC internal data structures remember that a number +of <TT>GC_</tt><I>xxx</i> variables are really macro defined to +<TT>GC_arrays._</tt><I>xxx</i>, so that +the collector can avoid scanning them. +</ol> +</body> +</html> + + + + |