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pcwalton's profile
Patrick Walton
Patrick Walton
Patrick Walton
@pcwalton

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Patrick Walton

@pcwalton

Research engineer at Mozilla

San Francisco, CA
pcwalton.github.io
Joined November 2009

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    Patrick Walton‏ @pcwalton 16 Sep 2019
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    I think a lot of people don’t realize just how fast nursery allocations are in optimized GC implementations. Here’s the generated code for malloc in the HotSpot JVM. (Source: https://umumble.com/blogs/java/how-does-jvm-allocate-objects%3F/ …)pic.twitter.com/8yj2vGsjZQ

    11:42 AM - 16 Sep 2019
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    • Robert Jack 啸风 Will isiah meadows 🧢 Jakub Wieczorek Praveeno This Dot Labs Jarred Nicholls Roman Grebennikov Vivitsu Maharaja Filip Haglund
    6 replies 29 retweets 92 likes
      1. Patrick Walton‏ @pcwalton 16 Sep 2019
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        Tuning Java or JS code to avoid allocations can sometimes be useful, but you aren’t likely to gain much if any *throughput* from it. The most you can expect is reducing the GC pause frequency. (This is different in languages like Go or Rust that have relatively slow allocation.)

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      2. Patrick Walton‏ @pcwalton 16 Sep 2019
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        Replying to @TheMichaelBurge

        Someone should implement a language that heap-allocates all call frames (like SML/NJ does), allocates the nursery backwards, and stores the nursery high-water mark in RSP, just to blow people’s minds

        6 replies 0 retweets 15 likes
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      2. whitequark‏ @whitequark 16 Sep 2019
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        Replying to @pcwalton

        doesn't even have to be that optimized, it's also a pointer bump in OCaml

        1 reply 0 retweets 3 likes
      3. Patrick Walton‏ @pcwalton 16 Sep 2019
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        Replying to @whitequark

        Yeah, I mean “optimized” in the sense of “following best practices for GC”, not like micro-optimized.

        0 replies 0 retweets 4 likes
      4. End of conversation
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      2. Sebastian Sylvan‏ @ssylvan 16 Sep 2019
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        Replying to @pcwalton

        The problem is the indirect cost. Generational hypothesis needs some time to play out. If you GC sooner because you allocated heavily, lots of objects that are actually very short lived may end up being copied to next generation because you didn't give the nursery enough time.

        1 reply 0 retweets 5 likes
      3. Sebastian Sylvan‏ @ssylvan 16 Sep 2019
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        Replying to @ssylvan @pcwalton

        So for e.g. games with GC (like Unity) the rule of thumb that "allocations are expensive" is still true, it's just not expensive where the allocation happens. Expensive indirectly and in aggregate.

        1 reply 0 retweets 3 likes
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      2. Gok‏ @Gok 16 Sep 2019
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        Replying to @pcwalton

        Eh who is claiming that the allocation is slow? I think more people are worried about the...garbage collecting part, and the required sand in the gears like read/write barriers.

        1 reply 0 retweets 0 likes
      3. Gok‏ @Gok 16 Sep 2019
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        Replying to @Gok @pcwalton

        Oh I see your second tweet explains the context much better :)

        0 replies 0 retweets 0 likes
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      2. Daniel HB‏ @DanielHoffmann_ 17 Sep 2019
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        Replying to @pcwalton

        not super knowledgable about the subject, but I always thought stack allocation was better because of less cache-misses and that actual allocation time was always mostly irrelevant.

        1 reply 0 retweets 0 likes
      3. Daniel HB‏ @DanielHoffmann_ 17 Sep 2019
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        Replying to @DanielHoffmann_ @pcwalton

        non-GC languages also need to pause when deallocating memory because it sometimes gets too fragmented, but you can control when that happens. So GC is not slower, just less deterministic.

        1 reply 0 retweets 0 likes
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