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marcan42's profile
Hector Martin
Hector Martin
Hector Martin
@marcan42

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Hector Martin

@marcan42

If it ain't broke, I'll fix it! I'm porting Linux to Apple Silicon Macs at @AsahiLinux. http://patreon.com/marcan  | http://github.com/sponsors/marcan 

Tokyo, Japan
marcan.st
Joined May 2009

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    1. comex‏ @comex 8 Jul 2020
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      comex Retweeted always @ ( * )

      Speculation is OK! There just has to be a way to reset *all* microarchitectural state before starting a test run, so speculation happens the same way every time. For DDR, you’d have to define a worst-case # of cycles for an access, then artificially delay all accesses to that.https://twitter.com/rzidane360/status/1280737724916355072 …

      comex added,

      always @ ( * ) @rzidane360
      Replying to @comex
      A deterministically timed CPU would be tough to build without severe limitations. Even if you build a completely in-order CPU with no speculation, the moment you have DDR involved, timing is not deterministic. DDR retraining/ZQ-calibration etc lead to big run-to-run differences.
      2 replies 0 retweets 8 likes
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    2. Hector Martin‏ @marcan42 8 Jul 2020
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      Replying to @comex

      This doesn't work. The problem is the worst case timings are *horrendous* and you would completely destroy performance if you tried to delay all accesses to match. 99th percentile latencies on modern systems are absurdly bad.

      1 reply 0 retweets 7 likes
    3. comex‏ @comex 8 Jul 2020
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      Replying to @marcan42

      Oh, is it? (Why?) I suppose you could go for more like 95th percentile and, if it takes longer, freeze the entire pipeline until ready, along with the cycle counter. A deterministic cycle count is good enough for benchmarking even if it doesn’t actually represent real time.

      1 reply 1 retweet 1 like
    4. Hector Martin‏ @marcan42 8 Jul 2020
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      Replying to @comex

      The DDR retraining creates a massive latency spike, during which all memory accesses are halted. Modern systems are usually fast, except when they're not, and then they're hilariously slow.

      2 replies 0 retweets 4 likes
    5. comex‏ @comex 8 Jul 2020
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      Replying to @marcan42

      I see...

      1 reply 0 retweets 0 likes
      Hector Martin‏ @marcan42 8 Jul 2020
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      Replying to @comex

      Also, a bunch of these things are *temperature-dependent*. Though at least that factor you can usually derate to "worst case" without an order of magnitude performance difference.

      2:27 AM - 8 Jul 2020
      0 replies 0 retweets 0 likes

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