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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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    Andres Freund (Tech)‏ @AndresFreundTec 30 Sep 2019
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    The often repeated recommendation to not exceed 8GB/25% of RAM for shared buffers in postgres is wrong. For pgbench scale 1500, on laptop with fast SSD and 32GB of RAM, parallelism of 16. s_b of 1GB, 8GB, 16GB, using huge pages. r/o: 115k, 100k, 185k r/w: 16300, 15100, 21500

    1:15 PM - 30 Sep 2019
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    6 replies 20 retweets 59 likes
      1. Andres Freund (Tech)‏ @AndresFreundTec 30 Sep 2019
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        And that's with a brief test. For the read write test, if the test is longer the differences become considerably starker.

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      2. Mark Callaghan‏ @MarkCallaghanDB 30 Sep 2019
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        Replying to @AndresFreundTec

        Thanks for the results. A few questions: 1) what OS? If Linux: 2) is any of this a result of kernel VM improvements? 3) for r/w is there more response time variance with 16gb because of writeback storms?

        1 reply 0 retweets 0 likes
      3. Andres Freund (Tech)‏ @AndresFreundTec 30 Sep 2019
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        Replying to @MarkCallaghanDB

        It's indeed linux. And I don't think it's really VM improvement depandant - I think this has been true for a long time. The response time for 24GB (see copy/paste correction upthread) is vastly better than for the other shared buffer settings, as there is no buffer replacement.

        2 replies 0 retweets 1 like
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      1. Andres Freund (Tech)‏ @AndresFreundTec 30 Sep 2019
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        Ugh, correction, I used 24GB, not 16GB. Stupid factor of two snuck in while tweeting.

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      2. Andres Freund (Tech)‏ @AndresFreundTec 30 Sep 2019
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        Replying to @codehovel

        It's due to double buffering leading to the whole working set fitting neither into kernel page cache, nor yet into postgres's shared_buffers. And it's close to the worst case for that, because there'll be substantial amounts of memory for that wasted, without too much benefit.

        0 replies 0 retweets 2 likes
      3. End of conversation
      1. Peter Eisentraut‏ @petereisentraut 1 Oct 2019
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        Replying to @AndresFreundTec

        So new recommendation: any value but 8GB?

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      1. Álvaro Hernández‏ @ahachete 1 Oct 2019
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        Replying to @AndresFreundTec

        It would be great if you drop this info as a comment on https://postgresqlco.nf/en/doc/param/shared_buffers … :)

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      1. Adrien Nayrat‏ @Adrien_nayrat 2 Oct 2019
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        Replying to @AndresFreundTec

        Interesting, I already faced buffer_mapping contention on a huge production. After several tests I wrote a quick & dirty tool to replay the workload to reproduce the issue. With smaller s_b it was worse and much better with bigger. I tried until 16Gb.

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