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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. Markus Winand‏ @MarkusWinand 19 Nov 2019
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      Anyone using PostgreSQL with 10k concurrent connections, most of them idle? Let's say PgBouncer is not an option. Experiences? Asking for a friend ;)

      9 replies 2 retweets 28 likes
    2. Dimitri Fontaine‏ @tapoueh 19 Nov 2019
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      Replying to @MarkusWinand

      There’s a patch on hackers that would do wonders with that situation (and others!). You can review and test https://commitfest.postgresql.org/25/2067/ 

      1 reply 0 retweets 5 likes
    3. Markus Winand‏ @MarkusWinand 19 Nov 2019
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      Replying to @tapoueh

      Thanks! Also: what are the actual problems one would face when doing it (with enough memory)?

      1 reply 0 retweets 0 likes
    4. Dimitri Fontaine‏ @tapoueh 19 Nov 2019
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      Replying to @MarkusWinand

      Every single lock needs to scan through 10k proc array entries, for starters.

      1 reply 0 retweets 3 likes
    5. Markus Winand‏ @MarkusWinand 19 Nov 2019
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      Replying to @tapoueh

      Yes, but how big is that problem actually? Do you know any measurements on that?

      1 reply 0 retweets 0 likes
    6. Dimitri Fontaine‏ @tapoueh 19 Nov 2019
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      Replying to @MarkusWinand

      Not yet. You’re going to share some is my understanding, with and without mentionnée patch, right?

      1 reply 0 retweets 1 like
    7. Markus Winand‏ @MarkusWinand 19 Nov 2019
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      Replying to @tapoueh

      Unfortunately, I won't :( I just try to figure out whether the common wisdom of too many connections is really bad is just legend, or if there are some actual war stories of what can/does go wrong then.

      1 reply 0 retweets 3 likes
    8. Dimitri Fontaine‏ @tapoueh 19 Nov 2019
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      Replying to @MarkusWinand

      It’s definitely not legend. I don’t have recent DBA work to share, good people to ask about more details include @pg_xocolatl and @RhodiumToad and @magnushagander and @AndresFreundTec I would think.

      1 reply 0 retweets 2 likes
    9. Andres Freund (Tech)‏ @AndresFreundTec 19 Nov 2019
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      Replying to @tapoueh @MarkusWinand and

      It's not a legend, indeed. It's not really lock entries that are a problem, however. There are some inefficiencies there, but not too bad. It's building snapshots for visibility determinations that is the problem. That needs to scan the procarray.

      1 reply 0 retweets 6 likes
      Andres Freund (Tech)‏ @AndresFreundTec 19 Nov 2019
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      Replying to @AndresFreundTec @tapoueh and

      Which is bad, because that's a very frequent task for a lot of workloads that use a lot of transaction - at the very least once a transaction (repeatable read), but more commonly at least two to three times (read committed).

      6:08 AM - 19 Nov 2019
      • 1 Like
      • Markus Winand
      1 reply 0 retweets 1 like
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        2. Andres Freund (Tech)‏ @AndresFreundTec 19 Nov 2019
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          Replying to @AndresFreundTec @tapoueh and

          You can quite easily see throughput drop w/ lots of active connections. And it gets a lot worse with bigger systems, as inter-socket traffic leads to significantly worse scalability behaviour.

          1 reply 0 retweets 2 likes
        3. Andres Freund (Tech)‏ @AndresFreundTec 19 Nov 2019
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          Replying to @AndresFreundTec @tapoueh and

          Some of this is inherent. Some of this is because we cause cachelines for the PGXACT/PGPROC entries to be dirtied a lot (leading to cachelines being bounced around), and the arrays containing this data not being scanned linearly (leading to poor pipelining, prefetching).

          0 replies 0 retweets 3 likes
        4. End of conversation

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