Future core architectures. (Was Haskell Compilation Improvement)

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), January 3, 2014 1:27 pm
Room: Moderated Discussions
Maynard Handley (name99.delete@this.name99.org) on January 3, 2014 12:06 am wrote:
>
> Does anyone have an opinion on how real this stuff is?

How much would you end up gaining on real loads? Not much, I think. Not taking the cost into account.

The thing is, fairly basic OoO already gives you most of the low hanging fruit. Nobody sane argues against OoO, sane people seem to look at ways to extend it a bit further without insanely big instruction windows. Thus the papers to try to do speculative multithreading etc.

Say you actually get 20% performance improvement from some fancy SpMT approach, and are able to use a couple of cores effectively to improve single-thread performance. Is that really worth it? Especially when a lot of environments have thermal or power limits that may well mean that a traditional single-core (non-SpMT) approach could instead just run at a higher frequency.

And that 20% performance improvement would be hard.

The rumors for Rock were that it was hugely power-hungry, for not much (any) performance advantage of the run-ahead. Speculation is expensive. People generally work very hard at not speculating - branch prediction, alias prediction for memory re-ordering etc etc.

For many integer loads that don't thread well, medium OoO is actually pretty good at capturing a reasonable amount of MLP (and for loads that are amenable to threading, SpMT sounds like a mistake). Huge instruction windows certainly help with some kinds of loads, but I think it's instructive how power-aware even the "big" CPU's have become. I'm not sure how sensible it is to expect speculation or other run-ahead models to become much more common.

Linus
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