--- /dev/null
+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=<bytes> - Initial heap size in bytes. May speed up
+ process start-up.
+
+GC_MAXIMUM_HEAP_SIZE=<bytes> - 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=<n> - 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=<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> - 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=<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 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[=<n>] - (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".
+
+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.
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