By: Jukka Larja (roskakori2006.delete@this.gmail.com), May 21, 2022 10:23 pm
Room: Moderated Discussions
Brendan (btrotter.delete@this.gmail.com) on May 21, 2022 10:36 am wrote:
> How about we create an OS where unsupported instructions are emulated; so that you can run a new executable
> that uses AVX-512 on a crusty old Prescott Pentium 4 from 2005 without any problem (other than performance);
> and where a "some cores don't support AVX-512" problem becomes a performance issue (a minor addition to
> the "P cores are faster than E cores" performance issue you already have to deal with)?
In many important cases, P cores aren't actually any faster than E cores. P cores support SMT, so with enough threads they aren't going to run at 100 % performance, but around 60-65 % instead. That may still be significantly more than an E cores, but it's not a huge difference.
Yet, if you've read Alder Lake reviews, there are some programs that actually will run faster with E cores disabled. There are actually even some programs that run faster on single-CPU-chiplet AMD Zens (e.g. 8 core vs. 16 core), unless locked to run on just single chiplet on the multi-chiplet model. It's seldom a huge issue, but if performance delta was large, it likely would be much bigger.
-JLarja
> How about we create an OS where unsupported instructions are emulated; so that you can run a new executable
> that uses AVX-512 on a crusty old Prescott Pentium 4 from 2005 without any problem (other than performance);
> and where a "some cores don't support AVX-512" problem becomes a performance issue (a minor addition to
> the "P cores are faster than E cores" performance issue you already have to deal with)?
In many important cases, P cores aren't actually any faster than E cores. P cores support SMT, so with enough threads they aren't going to run at 100 % performance, but around 60-65 % instead. That may still be significantly more than an E cores, but it's not a huge difference.
Yet, if you've read Alder Lake reviews, there are some programs that actually will run faster with E cores disabled. There are actually even some programs that run faster on single-CPU-chiplet AMD Zens (e.g. 8 core vs. 16 core), unless locked to run on just single chiplet on the multi-chiplet model. It's seldom a huge issue, but if performance delta was large, it likely would be much bigger.
-JLarja