View Single Post
Old 2021-09-05, 08:21   #11
mackerel
 
mackerel's Avatar
 
Feb 2016
UK

23×5×11 Posts
Default

Quote:
Originally Posted by JWNoctis View Post
Off topic, but...it's been a while, and I'm kind of surprised that nobody here had brought up the fact that, for whatever reason, Alder Lake apparently won't have AVX-512 support in consumer/client-level processors, including the entire Core lineup.

Which to my understanding would have a non-trivial impact on performance for Prime95, for those who still intend to crunch on their personal machine and planning to buy one of those.
A possible explanation I've seen is that both cores need to share a common feature set. As threads get moved around you can't end up with a situation where code needs AVX-512, but it can't be provided by the E cores. The solution for now is to disable AVX-512 where you have E cores present. Unfortunately they have also said you can't get AVX-512 back by disable E cores and run only P cores. I had hoped something smarter could be done at OS level, where for example code needing AVX-512 would only be run on P cores and not touch the E cores at all.

For consumer level CPUs, for bigger FFTs were way into the region where ram bandwidth is limiting. In that scenario, the loss of AVX-512 is less of an impact, and having faster DDR5 speeds would probably result in a net increase to performance.

Where the loss of AVX-512 will be felt more is for those like me who focus on smaller FFTs that are not limited by ram speeds. The open question remains, just what sort of FMA performance can the E cores provide? It might be a case of quantity over quality to provide a possible net positive in performance.
mackerel is offline   Reply With Quote