help wrt semantics / primops for pure prefetches

Simon Marlow marlowsd at
Fri Nov 28 08:42:14 UTC 2014

Thanks for this.  In the copying GC I was using prefetching during the 
scan phase, where you do have a pretty good tunable knob for how far 
ahead you want to prefetch.  The only variable is the size of the 
objects being copied, but most tend to be in the 2-4 words range.  I did 
manage to get 10-15% speedups with optimal tuning, but it was a slowdown 
on a different machine or with wrong tuning, which is why GHC doesn't 
have any of this right now.

Glad to hear this can actually be used to get real speedups in Haskell, 
I will be less sceptical from now on :)


On 27/11/2014 10:20, Edward Kmett wrote:
> My general experience with prefetching is that it is almost never a win
> when done just on trees, as in the usual mark-sweep or copy-collection
> garbage collector walk. Why? Because the time from the time you prefetch
> to the time you use the data is too variable. Stack disciplines and
> prefetch don't mix nicely.
> If you want to see a win out of it you have to free up some of the
> ordering of your walk, and tweak your whole application to support it.
> e.g. if you want to use prefetching in garbage collection, the way to do
> it is to switch from a strict stack discipline to using a small
> fixed-sized queue on the output of the stack, then feed prefetch on the
> way into the queue rather than as you walk the stack. That paid out for
> me as a 10-15% speedup last time I used it after factoring in the
> overhead of the extra queue. Not too bad for a weekend project. =)
> Without that sort of known lead-in time, it works out that prefetching
> is usually a net loss or vanishes into the noise.
> As for the array ops, davean has a couple of cases w/ those for which
> the prefetching operations are a 20-25% speedup, which is what motivated
> Carter to start playing around with these again. I don't know off hand
> how easily those can be turned into public test cases though.
> -Edward
> On Thu, Nov 27, 2014 at 4:36 AM, Simon Marlow <marlowsd at
> <mailto:marlowsd at>> wrote:
>     I haven't been watching this, but I have one question: does
>     prefetching actually *work*?  Do you have benchmarks (or better
>     still, actual library/application code) that show some improvement?
>     I admit to being slightly sceptical - when I've tried using
>     prefetching in the GC it has always been a struggle to get something
>     that shows an improvement, and even when I get things tuned on one
>     machine it typically makes things slower on a different processor.
>     And that's in the GC, doing it at the Haskell level should be even
>     harder.
>     Cheers,
>     Simon
>     On 22/11/2014 05:43, Carter Schonwald wrote:
>         Hey Everyone,
>         in
>         <>
>         and
>         https://phabricator.haskell.__org/D350
>         <>
>         is some preliminary work to fix up how the pure versions of the
>         prefetch
>         primops work is laid out and prototyped.
>         However, while it nominally fixes up some of the problems with
>         how the
>         current pure prefetch apis are fundamentally borken,  the simple
>         design
>         in D350 isn't quite ideal, and i sketch out some other ideas in the
>         associated ticket #9353
>         I'd like to make sure  pure prefetch in 7.10 is slightly less broken
>         than in 7.8, but either way, its pretty clear that working out
>         the right
>         fixed up design wont happen till 7.12. Ie, whatever makes 7.10,
>         there
>         WILL have to be breaking changes to fix those primops for 7.12
>         thanks and any feedback / thoughts appreciated
>         -Carter
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