By: Exophase (exophase.delete@this.gmail.com), May 9, 2013 8:57 pm
Room: Moderated Discussions
Brett (ggtgp.delete@this.yahoo.com) on May 9, 2013 8:26 pm wrote:
> Everything below servers has more cores than work to keep them busy.
Intel sells a lot of 2C/4T CPUs today and that's not stopping with Haswell (and I doubt it will with Broadwell). It's not a great stretch for desktops to make good use of > 2 hardware threads, even most games do so now.
They also charge a pretty big premium for going from 4C/4T to 4C/8T (along with a tiny clock bump and an L3 cache increase, but those usually have much less real world impact). I think they value it more than you realize. This could perhaps change if they go 6C or 8C with mainstream parts, but I don't see this becoming standard for a while yet.
> The last thing you want is to have
> a critical path task slowed down 50% because the task scheduler decided to throw more work on that CPU.
That's why OSes were made SMT aware, so they favor real cores first and try to keep them away from higher priority threads.
> Even in server land actual use of SMT is a niche market limited to certain databases and a few other tasks.
What are you saying, that you think most Intel servers turn SMT off? If you think that it often results in a performance hit those days are long gone.
> Anyone want to start a betting pool on when Intel dumps SMT for mainstream CPU's?
> The upcoming Tock, or the one after that? I guess you could also drop SMT on a Tick update.
>
Sure, I'll bet against it happening with Skylake and even more so with Skymont (I don't think Intel will get in the habit of removing major features at tocks). In fact, I have a feeling it could make a comeback with the 14nm or 10nm Atom tock. Although I can see why it was left out of Silvermont, it probably doesn't leave a lot of decode bandwidth on the table for a second thread and perhaps its first stage branch predictor wouldn't like the competition.
> Everything below servers has more cores than work to keep them busy.
Intel sells a lot of 2C/4T CPUs today and that's not stopping with Haswell (and I doubt it will with Broadwell). It's not a great stretch for desktops to make good use of > 2 hardware threads, even most games do so now.
They also charge a pretty big premium for going from 4C/4T to 4C/8T (along with a tiny clock bump and an L3 cache increase, but those usually have much less real world impact). I think they value it more than you realize. This could perhaps change if they go 6C or 8C with mainstream parts, but I don't see this becoming standard for a while yet.
> The last thing you want is to have
> a critical path task slowed down 50% because the task scheduler decided to throw more work on that CPU.
That's why OSes were made SMT aware, so they favor real cores first and try to keep them away from higher priority threads.
> Even in server land actual use of SMT is a niche market limited to certain databases and a few other tasks.
What are you saying, that you think most Intel servers turn SMT off? If you think that it often results in a performance hit those days are long gone.
> Anyone want to start a betting pool on when Intel dumps SMT for mainstream CPU's?
> The upcoming Tock, or the one after that? I guess you could also drop SMT on a Tick update.
>
Sure, I'll bet against it happening with Skylake and even more so with Skymont (I don't think Intel will get in the habit of removing major features at tocks). In fact, I have a feeling it could make a comeback with the 14nm or 10nm Atom tock. Although I can see why it was left out of Silvermont, it probably doesn't leave a lot of decode bandwidth on the table for a second thread and perhaps its first stage branch predictor wouldn't like the competition.
