What happens when DRAM has more bandwidth than Layer 3 cache?

By: Andrey (andrey.semashev.delete@this.gmail.com), December 8, 2022 3:10 pm
Room: Moderated Discussions
Etienne (etienne_lorrain.delete@this.yahoo.fr) on December 8, 2022 6:20 am wrote:
> Looks like my AMD Ryzen 9 7950x has a L3 cache bandwidth of 63.9 GB/s, my current DRAM DDR5
> has either 49.6 GB/s (Jedec) or 52.5 GB/s (AMD Expo) measured by memtest86 UEFI.
> It seems some companies are increasing DRAM bandwidth: 8Gbps DDR5.
>
> I assume latency to L3 cache is still probably better than latency to
> DRAM, but in simple terms, do we still need L3 cache in processors?

I don't know about Ryzen, but on Alder Lake memtest86 doesn't measure RAM/cache bandwidth anywhere close to other benchmarks, like AIDA64 or Intel Memory Latency Checker. It also doesn't read the current RAM speed and primary timings (I assume, it just always shows the first JEDEC profile), so I'm skeptical to any information it shows.

Here are AIDA64 results for 7700X, I expect L3 cache speeds to be comparable with 7950X. You can see ~900 GB/s for L3 reads, which is an order of magnitude higher than DRAM bandwidth, so I would say L3 is very much justified.

Your 52.5 GB/s is kinda low for a decent DDR5. Other tests, including the one I linked, also show lower speeds compared to Alder Lake and Raptor Lake. For example, depending on timings, you could get DDR5-6000 to show 92-97 GB/s for reads, 88-95 for writes and 62-65 ns latency on Alder Lake. Looks like Zen 4 is poorly optimized wrt. memory performance at this point. Maybe updated AGESA will improve things in the future.
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TopicPosted ByDate
What happens when DRAM has more bandwidth than Layer 3 cache?Etienne2022/12/08 06:20 AM
  What happens when DRAM has more bandwidth than Layer 3 cache?Simon Farnsworth2022/12/08 07:22 AM
    bandwidth*delay productMichael S2022/12/08 08:06 AM
    What happens when DRAM has more bandwidth than Layer 3 cache?---2022/12/08 10:44 AM
      What happens when DRAM has more bandwidth than Layer 3 cache?blaine2022/12/08 05:07 PM
  What happens when DRAM has more bandwidth than Layer 3 cache?Michael S2022/12/08 07:32 AM
    What happens when DRAM has more bandwidth than Layer 3 cache?Etienne2022/12/08 08:05 AM
      What happens when DRAM has more bandwidth than Layer 3 cache?Michael S2022/12/08 08:13 AM
        What happens when DRAM has more bandwidth than Layer 3 cache?Etienne2022/12/08 01:56 PM
  What happens when DRAM has more bandwidth than Layer 3 cache?Peter E. fry2022/12/08 08:20 AM
  Programs do not see bandwidth. Programs only see latency. Heikki Kultala2022/12/08 08:26 AM
    Programs do not see bandwidth. Programs only see latency. Chester2022/12/08 11:07 AM
  What happens when DRAM has more bandwidth than Layer 3 cache?Doug S2022/12/08 09:31 AM
  What happens when DRAM has more bandwidth than Layer 3 cache?---2022/12/08 10:32 AM
    What happens when DRAM has more bandwidth than Layer 3 cache?Michael S2022/12/08 02:42 PM
      What happens when DRAM has more bandwidth than Layer 3 cache?---2022/12/08 03:54 PM
        What happens when DRAM has more bandwidth than Layer 3 cache?Anon2022/12/20 06:46 AM
  What happens when DRAM has more bandwidth than Layer 3 cache?Andrey2022/12/08 03:10 PM
    What happens when DRAM has more bandwidth than Layer 3 cache?Etienne2022/12/14 03:20 AM
  What happens when DRAM has more bandwidth than Layer 3 cache?Gionatan Danti2022/12/09 12:31 AM
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