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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. DanielMicay‏ @DanielMicay Aug 27

      One of the highest priorities for a perf-oriented allocator should be keeping the working set of memory small by minimizing internal / external fragmentation and reducing memory held in caches. Understanding real world performance well is why jemalloc is being adopted everywhere.

      1 reply 2 retweets 3 likes
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    2. DanielMicay‏ @DanielMicay Aug 27

      The jemalloc design choices are bad for security, but it's an incredibly good allocator for long-term performance in everything but small programs. It has a strong focus on having minimal fragmentation and keeping other waste (metadata) at a tiny percentage of allocated memory.

      1 reply 1 retweet 1 like
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    3. DanielMicay‏ @DanielMicay Aug 27

      Google's TCMalloc was too concerned with performance of the allocator rather than applications and systems as a whole. Android chose jemalloc as the dlmalloc replacement despite jemalloc being maintained by Facebook and being somewhat more oriented towards server workloads.

      1 reply 0 retweets 0 likes
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    4. DanielMicay‏ @DanielMicay Aug 27

      I do think thread caches make sense most allocators, but they can accomplish nearly their entire purpose without being larger than arrays of about 16 pointers per size class, to reducing locking overhead to 1/8 (a simple approach is fill half when empty, flush half when full).

      2 replies 0 retweets 0 likes
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    5. Rich Felker‏ @RichFelker Aug 27
      Replying to @DanielMicay

      Is part of the problem perhaps that people writing them are thinking about scaling to 1024 cores or something?

      1 reply 0 retweets 0 likes
    6. Rich Felker‏ @RichFelker Aug 27
      Replying to @RichFelker @DanielMicay

      The lock cost you're trying to amortize should scale with the number of cores due to scaling of cache coherency traffic/contention, no?

      1 reply 0 retweets 0 likes
    7. DanielMicay‏ @DanielMicay Aug 27
      Replying to @RichFelker

      It might scale up somewhat but at least in jemalloc there are arenas assigned to threads either via round-robin (static assignment) or dynamic load balancing (sched_getcpu is the naive way, but Facebook has some kind of sophisticated replacement). The arenas what gives scaling.

      1 reply 0 retweets 0 likes
    8. DanielMicay‏ @DanielMicay Aug 27
      Replying to @DanielMicay @RichFelker

      Thread caches batch together a bunch of work which results in holding the locks for much longer, especially if the thread caches are large. They're kept quite small in jemalloc and have non-trivial garbage collection / heuristics esp compared to the massive ones used by TCMalloc.

      1 reply 0 retweets 1 like
    9. DanielMicay‏ @DanielMicay Aug 27
      Replying to @DanielMicay @RichFelker

      In TCMalloc, it was a bandaid for not having an efficient underlying allocator. It's a lot better for the throughput to come from a very high performance allocator behind the locks with the thread caches only needing to make small batches of work to wipe out locking costs.

      2 replies 0 retweets 0 likes
    10. Rich Felker‏ @RichFelker Aug 27
      Replying to @DanielMicay

      In any case your thread is interesting because rewriting malloc is on the mid-term agenda for musl. Our goals don't match entirely but have a lot of overlap.

      1 reply 0 retweets 1 like
      Rich Felker‏ @RichFelker Aug 27
      Replying to @RichFelker @DanielMicay

      And I think you're very right that heavy thread caching is a bad idea, and that organizing storage by size classes is smart.

      5:43 PM - 27 Aug 2018
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