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

[Christopher Allen]

Well, there are cases where even with single-threading you want a memory
barrier to prevent the CPU reordering instructions, but the shift to a
single-threaded runtime should elide _some_ locks expressly designed to
cope with multithreading.

On Sun, Jan 24, 2016 at 5:04 PM, Thomas Koster <tkoster at gmail.com> wrote:

> Yuras,
>
> 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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>



-- 
Chris Allen
Currently working on http://haskellbook.com
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