[m-dev.] Tuning BoehmGC

[Paul Bone]

To tune the initial heap size that boehm uses (which affects the number of
collections that occur at run time).  Use the environment variable
GC_INITIAL_HEAP_SIZE.  See doc/README.environment in the boehm_gc directory.

To tune the HBLKSIZE parameter that is used to size the thread local cache
amoung other things the C macros need to be defined:

    HBLKSIZE - measured in bytes.
    CPP_LOG_HBLKSIZE - log2(HBLKSIZE)

These need to be set together.  The default is 4K, I've tested 4, 8, 16 and 32
in a asm_fast.gc.par grade.  Larger values are better.  This is easy to
configure by adding it to the BOEHM_CFLAGS_FOR_THREADS variable in
Mmake.common.in in the Mercury distribution.  For example:

BOEHM_CFLAGS_FOR_THREADS = @ENABLE_BOEHM_THREAD_LOCAL_ALLOC@ \                                                
    @ENABLE_BOEHM_PARALLEL_MARK@ \                                                                            
    -DHBLKSIZE=8192 -DCPP_LOG_HBLKSIZE=13

I havn't tried this on a non-parallel grade yet.

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