[Haskell-cafe] concurrency vs. I/O in GHC

Andrew Coppin andrewcoppin at btinternet.com
Wed Oct 27 17:09:52 EDT 2010

On 27/10/2010 05:00 PM, John Lato wrote:
> I am somewhat surprised that all capabilities must be ready for GC; I 
> thought with the parallel GC that wouldn't be necessary.  But I don't 
> know much about GC implementations so I try not to let their behavior 
> surprise me too much.

GHC has a _parallel_ GC implementation, meaning that the GC event runs 
in parallel on several cores. But it does not (yet) have _concurrent_ 
GC, meaning that a GC event can happen at the same time as Haskell 
threads are running. (Basically, which Haskell code running, the 
references between objects could change while the GC engine is trying to 
analyse them, which would be Bad.) I understand that the developers are 
actively working on fixing this, since it can sometimes have a 
significant effect on the performance of multicore programs...

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