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AndresFreundTec's profile
Andres Freund (Tech)
Andres Freund (Tech)
Andres Freund (Tech)
@AndresFreundTec

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Andres Freund (Tech)

@AndresFreundTec

Postgres developer, working at Microsoft. Also: politics nerd, expat German in the US. Account for tech related things. For politics: @AndresFreundPol

San Francisco, CA
Joined August 2017

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    1. Jens Axboe‏ @axboe May 20
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      Replying to @AndresFreundTec

      It'd be trivial to ensure things are correct and eliminate any need to think about locking, which is always a win on the application side imho. I'll have to think about it a bit.

      2 replies 0 retweets 1 like
    2. Andres Freund (Tech)‏ @AndresFreundTec May 20
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      Replying to @axboe

      Cool! Random update: FWIW, got PG to be competitive with io_uring based AIO for several workloads (but file appending is really bad right now). But only when using DIO, buffered has considerably higher CPU usage/contention.

      2 replies 0 retweets 1 like
    3. Andres Freund (Tech)‏ @AndresFreundTec May 20
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      Replying to @AndresFreundTec @axboe

      The slowdown for buffered IO is noticeable both when the data is in the page cache (because submission lock is held while copying all the data), and when not (the number of io-wq workers causing CPU overhead, partially due to contention).

      1 reply 0 retweets 0 likes
    4. Jens Axboe‏ @axboe May 20
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      Replying to @AndresFreundTec

      For the read side, it should be no slower than read(2) eg if cached, but if you need locking held over the copy, then that's definitely a concern. For read side buffered IO, my goal is to make this work through page waitqueue callbacks and eliminate the need for io-wq.

      2 replies 0 retweets 0 likes
    5. Andres Freund (Tech)‏ @AndresFreundTec May 20
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      Replying to @axboe

      Buffered reads via io-u that need to actually execute IO cause kernel-side contention, particularly around bringing the page into page cache. Re waitqueue callbacks: That sounds awesome.

      1 reply 0 retweets 1 like
    6. Andres Freund (Tech)‏ @AndresFreundTec May 20
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      Replying to @AndresFreundTec

      pic.twitter.com/bMsXI0FikG

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    7. Jens Axboe‏ @axboe May 23
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      Replying to @AndresFreundTec

      You should re-run that test case with my async-buffered.4 branch pulled in!

      1 reply 0 retweets 1 like
    8. Andres Freund (Tech)‏ @AndresFreundTec May 23
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      Replying to @axboe

      Will try to. Really need to get another decent drive that I can pass through a VM to do testing like this without having to reboot my workstation.

      1 reply 0 retweets 1 like
    9. Andres Freund (Tech)‏ @AndresFreundTec May 23
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      Replying to @AndresFreundTec @axboe

      Also just generally for development of the aio (on linux using io-uring) patch for postgres. The Samsung 970 Pro I'm currently using for that isn't bad, but it's quite possible to reach its limits. And the performance is a bit too variable over time for benchmarking.

      1 reply 0 retweets 0 likes
    10. Johannes Truschnigg‏ @JohTru May 24
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      Replying to @AndresFreundTec @axboe

      Did you try overprovisioning the drive (by limiting the available space via either NVME namespacing, or partitioning the pre-defined namespace, if the device doesn't support namespace mgmt)? That solved most performance consistency problems for us with consumer devices.

      1 reply 0 retweets 0 likes
      Andres Freund (Tech)‏ @AndresFreundTec May 24
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      Replying to @JohTru @axboe

      That helped with other disks, but in this case it doesn't seem to be the main problem. I suspect it's a question of how much of the drives internal RAM is available for write caching,, and when it flushes that back.

      3:08 PM - 24 May 2020
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