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README.environment
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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 (by GC_dump_named) 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_COLLECT_AT_MALLOC=<n> - Override the default value specified by
GC_COLLECT_AT_MALLOC macro. Has no effect unless
GC is built with GC_COLLECT_AT_MALLOC defined.
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 allocator 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 allocator 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 n-th warning about very large
block allocations, starting with the n-th 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.
GC_RETRY_SIGNALS - Try to compensate for lost
thread suspend and restart signals (Pthreads only).
On by default for OSF1 (Tru64) or if the library is
sanitized, off otherwise. 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. "0" value means "do not retry signals".
GC_USE_GETWRITEWATCH=<n> - Only if MPROTECT_VDB and (GWW_VDB or SOFT_VDB) are
both defined (Win32 and Linux only). Explicitly specify
which strategy of keeping track of dirtied pages should
be used. If n=0, then fall back to protecting pages and
catching memory faults strategy), else the collector
tries to use GetWriteWatch-based strategy (GWW_VDB) or
soft-dirty bits strategy (SOFT_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. This is intended to be transparent, but
in case of the mprotect-based implementation it may cause
unintended system call failures (thus, use it with
caution).
GC_PAUSE_TIME_TARGET - Set the desired garbage collector pause time in
milliseconds (ms). 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. Any value, except for the given
special one, disables parallel marker (almost fully) for
now.
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 threshold of memory blocks unmapping
(the number of sequential garbage collections during those
a candidate block for unmapping should be marked as 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.