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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    1. Kelly Sommers‏ @kellabyte Apr 11

      I wonder how many people need milliseconds in time stamps because they need the real sub second time or if they record milliseconds because they need the happens-before history. They are different problems. The latter can be solved with an atomic counter at a much faster rate.

      22 replies 10 retweets 91 likes
    2. Rich Felker‏ @RichFelker Apr 12
      Replying to @kellabyte

      Why do you say atomic counter is faster? It might be for low contention but it likely scales worse. ns-resolution timestamp is cheap on modern systems.

      1 reply 0 retweets 0 likes
    3. Kelly Sommers‏ @kellabyte Apr 12
      Replying to @RichFelker

      How so? Gettimeofday() takes 40ns.

      2 replies 0 retweets 1 like
    4. Elazar Leibovich‏ @elazarl Apr 12
      Replying to @kellabyte @RichFelker

      You want rdtsc/p instead.

      2 replies 0 retweets 0 likes
    5. Rich Felker‏ @RichFelker Apr 12
      Replying to @elazarl @kellabyte

      clock_gettime is just rdtsc/p plus the arithmetic needed to adjust it to be meaningful as a timestamp, on systems where tsc is usable as a time source. If it's not you'll likely incur a syscall, but then using rdtsc directly isn't safe for cross-core event sequencing either.

      1 reply 0 retweets 0 likes
    6. Elazar Leibovich‏ @elazarl Apr 12
      Replying to @RichFelker @kellabyte

      Intel recommends to use rdtsc instead of HPET, HPET can't go back. Even my laptop has it. If I read it right→safe for x-core seq clock_gettime use vdso, but will function call, and might branch IIRC, if she cares about ns, she can just use rdtsc. Main problem IMHO is portabilitypic.twitter.com/Bcl0CmR6DS

      1 reply 0 retweets 0 likes
    7. Elazar Leibovich‏ @elazarl Apr 13
      Replying to @elazarl @RichFelker @kellabyte

      and of course, feel free to correct me if you have better sources. AFAIK rdtsc can and is used for sequencing, e.g., for ftrace events.

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker Apr 13
      Replying to @elazarl @kellabyte

      It depends on the cpu model. On some (older ones) it's not safe.

      5:42 AM - 13 Apr 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Elazar Leibovich‏ @elazarl Apr 13
          Replying to @RichFelker @kellabyte

          Yeah, I know that. But on every reasonable hardware that @kellabyte uses, it would be OK. See my four years old laptop. It's a perfectly reasonable assumption to make IMHO. Here's Ivy-bridge from 2012. Surprised to see modern server w/o it. For'em slower alternative availablepic.twitter.com/xCHdBrWGwf

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Apr 13
          Replying to @elazarl @kellabyte

          FYI the only x86's not affected by Spectre seem to lack invariant tsc. You should never hard-code unsafe assumptions like this that will break critical invariants if they're not met.

          1 reply 0 retweets 0 likes
        4. Rich Felker‏ @RichFelker Apr 13
          Replying to @RichFelker @elazarl @kellabyte

          Use the proper OS facilities that wrap the functionality with safe fallbacks or probe the OS for whether tsc is invariant & fallback or refuse to run if it's not.

          1 reply 0 retweets 0 likes
        5. Elazar Leibovich‏ @elazarl Apr 14
          Replying to @RichFelker @kellabyte

          Proper OS facilities are worse than useless here, since they do not guarantee what you really want, and don't guarantee certain latency, which @kellabyte need Proper CPU facilities are useful here, and I perfectly agree that adding hard failure for CPUID w/o inv TSC is a must

          1 reply 0 retweets 0 likes
        6. Elazar Leibovich‏ @elazarl Apr 14
          Replying to @elazarl @RichFelker @kellabyte

          Not entirely sure why does "x86 not affected by Spectre" relevant here. In my experience nowadays non-inv TSC CPUs in server context are pretty uncommon. Of course some use cases (e.g., widely used library) can't assume invTsc rdtsc, or even rdtsc at all.

          0 replies 0 retweets 0 likes
        7. End of conversation

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