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 GC logging.  Not functional with SMALL_CONFIG.

GC_LOG_FILE - The name of the log file.  Stderr by default.

GC_PRINT_VERBOSE_STATS - Turn on even more logging.

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> - 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 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_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 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_FULL_FREQUENCY - Set the desired number of partial collections between full
                  collections.  Matters only if GC_incremental is set.

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_ALL_INTERIOR_POINTERS - Turns on GC_all_interior_pointers and thus interior
                           pointer recognition.

GC_DONT_GC - Turns off garbage collection.  Use cautiously.

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.
