Towards concurrent garbage collection for GHC

Simon Marlow of Microsoft Research recently published a blog post entitled First results from GHC's new garbage collector. As his beautiful graphs show so clearly, this is a first step towards concurrent garbage collection. The blog post describes this advancement entirely from the perspective of throughput because the ability to collect per-thread nursery generations independently removes some of the blocking that was wasting mutator time in the previous version.

However, we believe that concurrent programming may become a killer application domain for Haskell and, in that context, latency can be critical. If GHC's garbage collector is made more concurrent, by allowing the old generation to be collected independently as well, then pause times could be greatly reduced and Haskell would have a considerable advantage over competing technologies like .NET.

We have found that even the best-behaved .NET programs that allocate still suffer GC pauses of around 20ms, over an order of magnitude longer than the 600µs pause times indicated on the graphs for this new version of GHC. Real .NET applications that were not designed from the ground up to attain low latency suffer stalls lasting several seconds!


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