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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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    Rich Felker‏ @RichFelker 8 Jul 2016

    Idea for new VDSO feature: a function to query whether there are tasks waiting to run on the current cpu.

    11:49 AM - 8 Jul 2016
    • 1 Retweet
    • 1 Like
    • Justin Cormack sergey bratus
    4 replies 1 retweet 1 like
      1. New conversation
      2. Rich Felker‏ @RichFelker 8 Jul 2016
        Replying to @RichFelker

        Proposed usage case is mutexes deciding whether to spin for a while or immediately futex_wait.

        1 reply 0 retweets 1 like
      3. Rich Felker‏ @RichFelker 8 Jul 2016
        Replying to @RichFelker

        If there are tasks waiting, spinning for a mutex is wasting cpu time during which something else could be running.

        3 replies 0 retweets 0 likes
      4. Michael Foukarakis‏ @mfukar 8 Jul 2016
        Replying to @RichFelker

        yielding while spinning? I think that's not a good idea.

        2 replies 0 retweets 0 likes
      5. Rich Felker‏ @RichFelker 8 Jul 2016
        Replying to @mfukar

        No. Immediately using futex_wait instead of spinning for a while first.

        1 reply 0 retweets 0 likes
      6. Rich Felker‏ @RichFelker 8 Jul 2016
        Replying to @RichFelker @mfukar

        Spinning first is almost-pure-win (modulo slight electricity cost) if nothing else is waiting to run, pure-loss if something is.

        1 reply 0 retweets 0 likes
      7. Michael Foukarakis‏ @mfukar 8 Jul 2016
        Replying to @RichFelker

        not pure loss. Scheduling decides if it's a loss. What's the compelling argument for messing with scheduling?

        1 reply 0 retweets 0 likes
      8. Rich Felker‏ @RichFelker 8 Jul 2016
        Replying to @mfukar

        I think you're misunderstanding. Every spin for which you don't get the lock is wasted cpu cycles. Scheduler is irrelevant.

        1 reply 0 retweets 0 likes
      9. Rich Felker‏ @RichFelker 8 Jul 2016
        Replying to @RichFelker @mfukar

        If there was nothing else that wanted to use those cycles, no big deal. But if there was, you've incurred a cost.

        1 reply 0 retweets 0 likes
      10. 3 more replies
      1. Rich Felker‏ @RichFelker 8 Jul 2016
        Replying to @RichFelker

        Alternate idea with different abstraction: VDSO version of futex_wait that spins in userspace iff kernel thinks that's appropriate.

        0 replies 0 retweets 1 like
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      1. New conversation
      2. Arrigo Triulzi‏ @cynicalsecurity 8 Jul 2016
        Replying to @RichFelker

        @sergeybratus you could just resurrect Irix and all the per-CPU tools we had back then (e.g. "runon" to pin to a CPU).

        1 reply 0 retweets 1 like
      3. sergey bratus‏ @sergeybratus 8 Jul 2016
        Replying to @cynicalsecurity @RichFelker

        Arguably, OSes are intricate cocktails of features--the mix being more important than the ingredients :)

        1 reply 0 retweets 1 like
      4. Arrigo Triulzi‏ @cynicalsecurity 8 Jul 2016
        Replying to @sergeybratus @RichFelker

        one word: MULTICS :p

        1 reply 0 retweets 0 likes
      5. sergey bratus‏ @sergeybratus 8 Jul 2016
        Replying to @cynicalsecurity @RichFelker

        I was once told that every systems paper idea has been already published in the IBM Systems Journal :)

        1 reply 0 retweets 1 like
      6. Arrigo Triulzi‏ @cynicalsecurity 8 Jul 2016
        Replying to @sergeybratus @RichFelker

        and all ITsec is contained in Karger74…

        2 replies 1 retweet 0 likes
      7. Tweet unavailable
      8. sergey bratus‏ @sergeybratus 8 Jul 2016
        Replying to @4Dgifts @cynicalsecurity @RichFelker

        So, we can play "if you knew you'd end up on an uninhabited island, which papers you'd take along?" :)

        2 replies 0 retweets 0 likes
      9. Arrigo Triulzi‏ @cynicalsecurity 8 Jul 2016
        Replying to @sergeybratus @4Dgifts @RichFelker

        between the three of us? Can I just bring @daniel_bilar as a "big book"?

        1 reply 0 retweets 2 likes
      10. 1 more reply
      1. Carlos O'Donell‏ @CarlosODonell 9 Jul 2016
        Replying to @RichFelker

        Why?

        0 replies 0 retweets 0 likes
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