The garbage collector looks at a number of environment variables which are then used to affect its operation. These are examined only on Un*x-like platforms and win32. GC_INITIAL_HEAP_SIZE= - Initial heap size in bytes. May speed up process start-up. GC_MAXIMUM_HEAP_SIZE= - Maximum collected heap size. GC_LOOP_ON_ABORT - Causes the collector abort routine to enter a tight loop. This may make it easier to debug, such a process, especially for multithreaded platforms that don't produce usable core files, or if a core file would be too large. On some platforms, this also causes SIGSEGV to be caught and result in an infinite loop in a handler, allowing similar debugging techniques. GC_PRINT_STATS - Turn on as much logging as is easily feasible without adding signifcant runtime overhead. Doesn't work if the collector is built with SMALL_CONFIG. Overridden by setting GC_quiet. On by default if the collector was built without -DSILENT. GC_DUMP_REGULARLY - Generate a GC debugging dump GC_dump() on startup and during every collection. Very verbose. Useful if you have a bug to report, but please include only the last complete dump. GC_BACKTRACES= - Generate n random backtraces (for heap profiling) after each GC. Collector must have been built with KEEP_BACK_PTRS. This won't generate useful output unless most objects in the heap were allocated through debug allocators. This is intended to be only a statistical sample; individual traces may be erroneous due to concurrent heap mutation. GC_PRINT_ADDRESS_MAP - Linux only. Dump /proc/self/maps, i.e. various address maps for the process, to stderr on every GC. Useful for mapping root addresses to source for deciphering leak reports. GC_NPROCS= - Linux w/threads only. Explicitly sets the number of processors that the GC should expect to use. Note that setting this to 1 when multiple processors are available will preserve correctness, but may lead to really horrible performance, since the lock implementation will immediately yield without first spinning. GC_MARKERS= - Linux w/threads and parallel marker only. Set the number of marker threads. This is normaly set to the number of processors. It is safer to adjust GC_MARKERS than GC_NPROCS, since GC_MARKERS has no impact on the lock implementation. GC_NO_BLACKLIST_WARNING - Prevents the collector from issuing warnings about allocations of very large blocks. Deprecated. Use GC_LARGE_ALLOC_WARN_INTERVAL instead. GC_LARGE_ALLOC_WARN_INTERVAL= - Print every nth warning about very large block allocations, starting with the nth one. Small values of n are generally benign, in that a bounded number of such warnings generally indicate at most a bounded leak. For best results it should be set at 1 during testing. Default is 5. Very large numbers effectively disable the warning. GC_IGNORE_GCJ_INFO - Ignore the type descriptors implicitly supplied by GC_gcj_malloc and friends. This is useful for debugging descriptor generation problems, and possibly for temporarily working around such problems. It forces a fully conservative scan of all heap objects except those known to be pointerfree, and may thus have other adverse effects. GC_PRINT_BACK_HEIGHT - Print max length of chain through unreachable objects ending in a reachable one. If this number remains bounded, then the program is "GC robust". This ensures that a fixed number of misidentified pointers can only result in a bounded space leak. This currently only works if debugging allocation is used throughout. It increases GC space and time requirements appreciably. This feature is still somewhat experimental, and requires that the collector have been built with MAKE_BACK_GRAPH defined. For details, see Boehm, "Bounding Space Usage of Conservative Garbage Collectors", POPL 2001, or http://lib.hpl.hp.com/techpubs/2001/HPL-2001-251.html . GC_RETRY_SIGNALS, GC_NO_RETRY_SIGNALS - Try to compensate for lost thread suspend signals in linux_threads.c. On by default for GC_OSF1_THREADS, off otherwise. Note that this does not work around a possible loss of thread restart signals. This seems to be necessary for some versions of Tru64. Since we've previously seen similar issues on some other operating systems, it was turned into a runtime flag to enable last-minute work-arounds. GC_IGNORE_FB[=] - (Win32 only.) Try to avoid treating a mapped frame buffer as part of the root set. Certain (higher end?) graphics cards seems to result in the graphics memory mapped into the user address space as writable memory. Unfortunately, there seems to be no systematic way to identify such memory. Setting the environment variable to n causes the collector to ignore mappings longer than n MB. The default value of n is currently 15. (This should cover a 16 MB graphics card, since the mapping appears to be slightly shorter than all of graphics memory. It will fail if a dll writes pointers to collectable objects into a data segment whose length is >= 15MB. Empirically that's rare, but certainly possible.) WARNING: Security sensitive applications should probably disable this feature by setting GC_disallow_ignore_fb, or by building with -DNO_GETENV, since small values could force collection of reachable objects, which is conceivably a (difficult to exploit) security hole. GC_IGNORE_FB values less than 3 MB are never honored, eliminating this risk for most, but not all, applications. This feature is likely to disappear if/when we find a less disgusting "solution". IN VERSION 6.4 AND LATER, THIS SHOULD BE UNNECESSARY. The following turn on runtime flags that are also program settable. Checked only during initialization. We expect that they will usually be set through other means, but this may help with debugging and testing: GC_ENABLE_INCREMENTAL - Turn on incremental collection at startup. Note that, depending on platform and collector configuration, this may involve write protecting pieces of the heap to track modifications. These pieces may include pointerfree objects or not. Although this is intended to be transparent, it may cause unintended system call failures. Use with caution. GC_PAUSE_TIME_TARGET - Set the desired garbage collector pause time in msecs. This only has an effect if incremental collection is enabled. If a collection requires appreciably more time than this, the client will be restarted, and the collector will need to do additional work to compensate. The special value "999999" indicates that pause time is unlimited, and the incremental collector will behave completely like a simple generational collector. If the collector is configured for parallel marking, and run on a multiprocessor, incremental collection should only be used with unlimited pause time. GC_FIND_LEAK - Turns on GC_find_leak and thus leak detection. Forces a collection at program termination to detect leaks that would otherwise occur after the last GC. GC_ALL_INTERIOR_POINTERS - Turns on GC_all_interior_pointers and thus interior pointer recognition. GC_DONT_GC - Turns off garbage collection. Use cautiously.