parallel garbage collection performance

[Manuel M T Chakravarty]

Bryan O'Sullivan <bos at serpentine.com>:
> On Mon, Jun 18, 2012 at 9:32 PM, John Lato <jwlato at gmail.com> wrote:
> 
> I had thought the last core parallel slowdown problem was fixed a
> while ago, but apparently not?
> 
> Simon Marlow has thought so in the not too distant past (since he did the work), if my recollection is correct.

It may very well be fixed for non-dataparallel programs. For dataparallel programs the situation is more tricky as we rely on all threads participating in a DP computation to be scheduled simultaneously. If one Core is currently tied up by the OS, then GHC's RTS can't do anything about that.  As it has no concept of gang scheduling (but treats the threads participating in a DP computation individually), it also doesn't know that scheduling a subset of the threads in the gang is counterproductive.

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