By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), December 8, 2014 1:34 pm
Room: Moderated Discussions
Jouni Osmala (josmala.delete@this.cc.hut.fi) on December 8, 2014 1:10 pm wrote:
>
> I'm assuming that 90+% of programs already run fast enough and they don't matter for this.
> Its all about asking question in what use current computers are too slow , and can you parallerize
> that or are those cases already parallel. And I'm assuming you can parallerize atleast 10%
> of those times where user waits CPU for long enough to actually notice it.
What's the advantage?
You won't get scaling for much longer, and current trends are actually for lower power anyway. So what's the upside of pushing the whole parallelism snake-oil? We know that we need fairly complex OoO CPU's anyway, because people want reasonable performance and it turns out OoO is actually more efficient than slow in-order.
The whole "let's parallelize" thing is a huge waste of everybody's time. There's this huge body of "knowledge" that parallel is somehow more efficient, and that whole huge body is pure and utter garbage. Big caches are efficient. Parallel stupid small cores without caches are horrible unless you have a very specific load that is hugely regular (ie graphics).
Nobody is ever going to go backwards from where we are today. Those complex OoO cores aren't going away. Scaling isn't going to continue forever, and people want mobility, so the crazies talking about scaling to hundreds of cores are just that - crazy. Why give them an ounce of credibility?
Where the hell do you envision that those magical parallel algorithms would be used?
The only place where parallelism matters is in graphics or on the server side, where we already largely have it. Pushing it anywhere else is just pointless.
So give up on parallelism already. It's not going to happen. End users are fine with roughly on the order of four cores, and you can't fit any more anyway without using too much energy to be practical in that space. And nobody sane would make the cores smaller and weaker in order to fit more of them - the only reason to make them smaller and weaker is because you want to go even further down in power use, so you'd still not have lots of those weak cores.
So the whole argument that people should parallelise their code is fundamentally flawed. It rests on incorrect assumptions. It's a fad that has been going on too long.
Parallel code makes sense in the few cases I mentioned, where we already largely have it covered, because in the server space, people have been parallel for a long time.
It does not necessarily make sense elsewhere. Even in completely new areas that we don't do today because you cant' afford it. If you want to do low-power ubiquotous computer vision etc, I can pretty much guarantee that you're not going to do it with code on a GP CPU. You're likely not even going to do it on a GPU because even that is too expensive (power wise), but with specialized hardware, probably based on some neural network model.
Give it up. The whole "parallel computing is the future" is a bunch of crock.
Linus
>
> I'm assuming that 90+% of programs already run fast enough and they don't matter for this.
> Its all about asking question in what use current computers are too slow , and can you parallerize
> that or are those cases already parallel. And I'm assuming you can parallerize atleast 10%
> of those times where user waits CPU for long enough to actually notice it.
What's the advantage?
You won't get scaling for much longer, and current trends are actually for lower power anyway. So what's the upside of pushing the whole parallelism snake-oil? We know that we need fairly complex OoO CPU's anyway, because people want reasonable performance and it turns out OoO is actually more efficient than slow in-order.
The whole "let's parallelize" thing is a huge waste of everybody's time. There's this huge body of "knowledge" that parallel is somehow more efficient, and that whole huge body is pure and utter garbage. Big caches are efficient. Parallel stupid small cores without caches are horrible unless you have a very specific load that is hugely regular (ie graphics).
Nobody is ever going to go backwards from where we are today. Those complex OoO cores aren't going away. Scaling isn't going to continue forever, and people want mobility, so the crazies talking about scaling to hundreds of cores are just that - crazy. Why give them an ounce of credibility?
Where the hell do you envision that those magical parallel algorithms would be used?
The only place where parallelism matters is in graphics or on the server side, where we already largely have it. Pushing it anywhere else is just pointless.
So give up on parallelism already. It's not going to happen. End users are fine with roughly on the order of four cores, and you can't fit any more anyway without using too much energy to be practical in that space. And nobody sane would make the cores smaller and weaker in order to fit more of them - the only reason to make them smaller and weaker is because you want to go even further down in power use, so you'd still not have lots of those weak cores.
So the whole argument that people should parallelise their code is fundamentally flawed. It rests on incorrect assumptions. It's a fad that has been going on too long.
Parallel code makes sense in the few cases I mentioned, where we already largely have it covered, because in the server space, people have been parallel for a long time.
It does not necessarily make sense elsewhere. Even in completely new areas that we don't do today because you cant' afford it. If you want to do low-power ubiquotous computer vision etc, I can pretty much guarantee that you're not going to do it with code on a GP CPU. You're likely not even going to do it on a GPU because even that is too expensive (power wise), but with specialized hardware, probably based on some neural network model.
Give it up. The whole "parallel computing is the future" is a bunch of crock.
Linus
