[mercury-users] Parallelizing the garbage collector?

[Peter Ross]

On Tue, Mar 21, 2006 at 09:31:53PM +0100, Ondrej Bojar wrote:
> Hi.
> 
> I'm still hoping to use threads in my computations, but as I posted
> recently and Peter Ross suggested, Boehm garbage collector wastes far
> more time than the parallel processing can bring. After reading some of
> the documentation, I believe that the gc needs to stop all the threads
> too often to perform collection.
> 
> Boehm gc supports some extra flags to improve performance:
> -DPARALLEL_MARK for parallel marking and -DTHREAD_LOCAL_ALLOC for
> separate collectors for individual threads. (Parallel mark should help 
> straight away, thread local needs to be actually used in mmc.)
> 
My personal feeling, not having any evidence to back it up, is that
it's allocation not garbage collection which is slowing the system down.
I believe that the GC needs a lock to do allocation and because Mercury
is so allocation intensive, we effectively serialize the code, thus
for me getting THREAD_LOCAL_ALLOC is what is important.

In my reading of the documentation only linux support
THREAD_LOCAL_ALLOC, but that it doesn't require any support from the
compiler.  All it is is that each thread has their own free list to
allocate from, thus avoiding contention on the free list.

So I would concentrate on getting THREAD_LOCAL_ALLOC working.

> I tried to add these options to rotd/configure into
> CFLAGS_FOR_THREADS for my platform (linux, x64), but mmc compiled with
> these extra flags fails to link my programs:
> 
> /home/bojar/opt/mercury-compiler-rotd-2006-03-18-x64-gcthread/lib/mercury/lib/libpar_gc.a(alloc.o)(.text+0x5a5): 
> 
> In function `GC_set_fl_marks':
> : undefined reference to `GC_compare_and_exchange'
> /home/bojar/opt/mercury-compiler-rotd-2006-03-18-x64-gcthread/lib/mercury/lib/libpar_gc.a(mark.o)(.text+0xeb): 
> 
> In function `GC_set_mark_bit':
> : undefined reference to `GC_compare_and_exchange'
> ...
> 
> As far as I understand, boehm_gc/configure (which would support option 
> --enable-parallel-mark) is not called from generic configure, so I 
> cannot check if it would work better.
> 
> Would you recommend any workaround?
> 
>From the code, it looks like you need to define GENERIC_COMPARE_AND_SWAP
see pthreads_support.c and search for GC_compare_and_exchange.

Pete
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