Re: help wrt semantics / primops for pure prefetches

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 :) Cheers, Simon 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 mailto:marlowsd@gmail.com> 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 https://ghc.haskell.org/trac/__ghc/ticket/9353 https://ghc.haskell.org/trac/ghc/ticket/9353 and https://phabricator.haskell.__org/D350 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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