help wrt semantics / primops for pure prefetches

Gregory Collins greg at
Sun Nov 30 02:15:58 UTC 2014

On Thu, Nov 27, 2014 at 1:36 AM, Simon Marlow <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.

I've gotten some speedups (for some operations) from speculative prefetch
for hash tables -- e.g. for cuckoo hash where you know the key can be in
one of two buckets with p=0.5. Prefetching one while you search the other
lets you squeeze out some instruction-level parallelism, at the expense of
cache thrashing.

The cache thrashing issue means that whether prefetching works for you
depends a lot on your inputs: it can help if your program can handle some
additional cache pressure, and it might hurt you otherwise.

Gregory Collins <greg at>
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