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README.environment
The garbage collector looks at a number of environment variables which are, then, used to affect its operation. GC_INITIAL_HEAP_SIZE=<bytes> - Initial heap size in bytes. May speed up process start-up. Optionally, may be specified with a multiplier ('k', 'M' or 'G') suffix. GC_MAXIMUM_HEAP_SIZE=<bytes> - Maximum collected heap size. Allows a multiplier suffix. 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 multi-threaded 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 GC logging. Not functional with SMALL_CONFIG. GC_LOG_FILE - The name of the log file. Stderr by default. Not functional with SMALL_CONFIG. GC_ONLY_LOG_TO_FILE - Turns off redirection of GC stdout and stderr to the log file specified by GC_LOG_FILE. Has no effect unless GC_LOG_FILE is set. Not functional with SMALL_CONFIG. GC_PRINT_VERBOSE_STATS - Turn on even more logging. Not functional with SMALL_CONFIG. 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=<n> - Generate n random back-traces (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=<n> - 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=<n> - Only if compiled with PARALLEL_MARK. Set the number of marker threads. This is normally 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=<n> - 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 pointer-free, 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_USE_GETWRITEWATCH=<n> - Only if MPROTECT_VDB and GWW_VDB are both defined (Win32 only). Explicitly specify which strategy of keeping track of dirtied pages should be used. If n=0 then GetWriteWatch() is not used (falling back to protecting pages and catching memory faults strategy) else the collector tries to use GetWriteWatch-based strategy (GWW_VDB) first if available. GC_DISABLE_INCREMENTAL - Ignore runtime requests to enable incremental GC. Useful for debugging. 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 pointer-free 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_FULL_FREQUENCY - Set the desired number of partial collections between full collections. Matters only if GC_incremental is set. Not functional with SMALL_CONFIG. GC_FREE_SPACE_DIVISOR - Set GC_free_space_divisor to the indicated value. Setting it to larger values decreases space consumption and increases GC frequency. GC_UNMAP_THRESHOLD - Set the desired memory blocks unmapping threshold (the number of sequential garbage collections for which a candidate block for unmapping should remain free). The special value "0" completely disables unmapping. GC_FORCE_UNMAP_ON_GCOLLECT - Turn "unmap as much as possible on explicit GC" mode on (overrides the default value). Has no effect on implicitly-initiated garbage collections. Has no effect if memory unmapping is disabled (or not compiled in) or if the unmapping threshold is 1. 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_FINDLEAK_DELAY_FREE - Turns on deferred freeing of objects in the leak-finding mode (see the corresponding macro description for more information). GC_ABORT_ON_LEAK - Causes the application to be terminated once leaked or smashed objects are found. GC_ALL_INTERIOR_POINTERS - Turns on GC_all_interior_pointers and thus interior pointer recognition. GC_DONT_GC - Turns off garbage collection. Use cautiously. GC_USE_ENTIRE_HEAP - Set desired GC_use_entire_heap value at start-up. See the similar macro description in README.macros. GC_TRACE=addr - Intended for collector debugging. Requires that the collector have been built with ENABLE_TRACE defined. Causes the debugger to log information about the tracing of address ranges containing addr. Typically addr is the address that contains a pointer to an object that mysteriously failed to get marked. Addr must be specified as a hexadecimal integer.