By: Doug S (foo.delete@this.bar.bar), May 12, 2013 8:22 pm
Room: Moderated Discussions
mpx (mpx.delete@this.nomail.pl) on May 12, 2013 12:04 pm wrote:
> Doug S (foo.delete@this.bar.bar) on May 11, 2013 10:49 am wrote:
> > What the hell would people be doing on their phone that they need
> > 8 threads?
>
> A general characteristics of CPU - memory is high-latency - is the main driver for
> SMT. Smaprthones of the past were less bothered by it because of sub-1GHz frequencies.
> Now, with frequencies approaching 2GHz they would benefit from SMT more.
You still need to be actually doing something that maxes out your four real cores while still needing more resources. On a phone. I'm talking a real world task that would be done more than a fraction of the userbase, so no talk about a couple games that can theoretically be a tiny bit faster with 4C/8T versus 4C, or people running a browser with >4 tabs in a process-per-tab model where even the inactive ones are maxing out the CPU (Flash could do this, but luckily that abomination is rapidly dying)
I agree that as frequencies increase, you are more cycles away from main memory. But so what? Until you're doing more things that are maxing out all cores on human timescales SMT won't make any difference for the user experience.
Your argument works equally well to reason that 4 cores on a phone isn't enough, they should have 6, or 8, or 10. Sure SMT is less expensive than adding another real core, but it isn't free either because each core needs more execution resources than the it otherwise would have had, and more cache than it otherwise would have had, and another full register file. Adding a second core made a real difference to many people's workloads, both on desktops to a limited extent, on phones. Going to quad core is hardly noticeable except for niche uses, so there's no point in going beyond that except for marketing purposes.
> Doug S (foo.delete@this.bar.bar) on May 11, 2013 10:49 am wrote:
> > What the hell would people be doing on their phone that they need
> > 8 threads?
>
> A general characteristics of CPU - memory is high-latency - is the main driver for
> SMT. Smaprthones of the past were less bothered by it because of sub-1GHz frequencies.
> Now, with frequencies approaching 2GHz they would benefit from SMT more.
You still need to be actually doing something that maxes out your four real cores while still needing more resources. On a phone. I'm talking a real world task that would be done more than a fraction of the userbase, so no talk about a couple games that can theoretically be a tiny bit faster with 4C/8T versus 4C, or people running a browser with >4 tabs in a process-per-tab model where even the inactive ones are maxing out the CPU (Flash could do this, but luckily that abomination is rapidly dying)
I agree that as frequencies increase, you are more cycles away from main memory. But so what? Until you're doing more things that are maxing out all cores on human timescales SMT won't make any difference for the user experience.
Your argument works equally well to reason that 4 cores on a phone isn't enough, they should have 6, or 8, or 10. Sure SMT is less expensive than adding another real core, but it isn't free either because each core needs more execution resources than the it otherwise would have had, and more cache than it otherwise would have had, and another full register file. Adding a second core made a real difference to many people's workloads, both on desktops to a limited extent, on phones. Going to quad core is hardly noticeable except for niche uses, so there's no point in going beyond that except for marketing purposes.
