Re: [Haskell-cafe] Turn GC off

Thank you for the clarification. I had read those papers, but I was under the impression that it was something already part of GHC 7. Regards, Dave On Thu, Sep 29, 2011 at 8:45 PM, austin seipp wrote:

On Thu, Sep 29, 2011 at 3:14 PM, David Barbour wrote:

...

minor collections of this nursery do not result in whole system pauses.

Yes, they do. GHC has a parallel garbage collector (so collection pauses the mutator threads, and collects garbage -in parallel- on multiple CPUs) but it in no way has a concurrent one. Every OS thread has its own young-gen heap space, and the old-generation heap space is shared amongst every CPU. But any young-gen GC will cause all mutator threads to pause no matter what the state of the others is.

That said, Simon^2 has done research on this problem recently. They recently published a paper about 'local' garbage collection for individual OS threads, where every thread has its own, private nursery, that may be collected independently of all other CPUs, which promotes objects into the global heap when necessary for access from other threads. The design is reminiscent of older work on the same topic (thread-local heaps,) but adds a bunch of tasty work they did.

You can find this branch of GHC with 'local heaps' here, in the local-gc branch of the git repository:

https://github.com/ghc/ghc/tree/local-gc

This new local collector branch is not, I repeat, not part of GHC and hasn't been merged. It's not certain if it ever will be, I think. The paper conclusion addresses the fact the scalability improvements as a result of this new collector are nowhere near as dramatic as they had hoped, and it's not certain the improvements they did get are worth the substantial complexity increase. It doesn't address the old-gen GC - any old-gen GCs still pause all mutator threads before continuing. They do note however that this local allocation strategy could be combined with a real concurrent/incremental GC for the old-generation, which would help control pause times more over the whole lifetime of an application.

You can find all the juicy details in their paper "Multicore garbage collection with thread local heaps", located near the top of Simon's webpage here:

http://research.microsoft.com/en-us/people/simonpj/

-- Regards, Austin