Ampere Altra Max 16MB LLC with 128 cores

By: Chester (lamchester.delete@this.gmail.com), May 26, 2021 2:54 pm
Room: Moderated Discussions
Anon (no.delete@this.spam.com) on May 26, 2021 1:36 pm wrote:
> Heikki Kultala (heikki.kul.tala.delete@this.gmail.com) on May 25, 2021 11:22 pm wrote:
> > I'm not sure it even is a victim cache. As it's called "system level cache" it might be a
> > memory side cache which also caches accesses made by other accelerators and PCIe devices.
>
> Would be possible that those 16MB cache gives preference for
> cache lines evicted with any variant of "shared" state?
>
> I don't think it would be very useful if not.

AMD's L3 prefers shared state in a slightly different way - shared lines stay in L3 while other states don't. To be exact:
- L3 hit from store data access: line always invalidated from L3
- L3 hit from load data access: line invalidated if only one core read it (not shared state)
- L3 hit by more than one core: line stays valid in L3 (shared state)
- L3 hit from code fetch: line stays valid in L3 (also shared state I think)

Also data from non-temporal prefetches is not filled into L3 when evicted from L2. Contrary to what some places say, non-temporal has nothing to do with RFO and cache coherency. Non-temporal simply means temporal locality will be bad (data not likely to be used again soon) so don't waste cache space with it.

The only way you avoid RFOs is when you know you're overwriting an entire cache line. Examples include P6 using a non-RFO cache protocol for rep mov, and possibly avx-512 writes (though I couldn't find a source).
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TopicPosted ByDate
Ampere Altra Max 16MB LLC with 128 coresGanon2021/05/25 01:30 AM
  Ampere Altra Max 16MB LLC with 128 coresanon2021/05/25 03:11 AM
    Ampere Altra Max 16MB LLC with 128 coresHeikki Kultala2021/05/25 11:22 PM
      Ampere Altra Max 16MB LLC with 128 coresAnon2021/05/26 01:36 PM
        Ampere Altra Max 16MB LLC with 128 coresChester2021/05/26 02:54 PM
          Ampere Altra Max 16MB LLC with 128 coresChester2021/05/26 03:03 PM
  Ampere Altra Max 16MB LLC with 128 coresDoug S2021/05/25 07:50 AM
    Ampere Altra Max 16MB LLC with 128 coresAndrei F2021/05/25 08:06 AM
    Ampere Altra Max 16MB LLC with 128 coresRayla2021/05/25 08:17 AM
  A few thoughts on Ampere's Altra MaxPaul A. Clayton2021/05/27 12:00 PM
    A few thoughts on Ampere's Altra MaxBjörn Ragnar Björnsson2021/05/27 03:47 PM
      Yeah, I should have looked for and through a data sheet (NT)Paul A. Clayton2021/05/27 06:25 PM
    A few thoughts on Ampere's Altra MaxAdrian2021/05/27 11:13 PM
      Boring can be profitablePaul A. Clayton2021/05/29 12:18 PM
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