[Haskell-cafe] When are MVars better than STM?

Thomas Koster tkoster at gmail.com
Sun Jan 24 23:04:27 UTC 2016


On Sun, 2016-01-24 at 17:46 +1100, Thomas Koster wrote:
> 2. When given two capabilities (+RTS -N2), MVars are suddenly an
> order
> of magnitude slower than with just one capability. Why?

On 25 January 2016 at 02:09, Yuras Shumovich <shumovichy at gmail.com> wrote:
> One possible explanation is closure locking which is not performed when
> there is only one capability. In my quick measurements it gives 40%
> speedup: https://ghc.haskell.org/trac/ghc/ticket/693#comment:9

This makes sense. After all, why bother with locks and barriers when
the process is single-threaded anyway?

Thanks for your response.

Thomas Koster

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