On Thu, Nov 27, 2014 at 1:36 AM, Simon Marlow
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.
G
--
Gregory Collins