Lemmy Server Performance

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lemmy_server uses the Diesel ORM that automatically generates SQL statements. There are serious performance problems in June and July 2023 preventing Lemmy from scaling. Topics include caching, PostgreSQL extensions for troubleshooting, Client/Server Code/SQL Data/server operator apps/sever operator API (performance and storage monitoring), etc.

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Lemmy PostgreSQL performance: comment_like & post_like tables have a lot of indexes (5 alone on comment_like), SQL INSERT is ~~taking 0.4 seconds per upvote.~~ Federation incoming lower priority? (lemmy.ml)

submitted 1 year ago* (last edited 1 year ago) by [email protected] to c/[email protected]

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Federation likes (votes) are wildly different from server to server as it stands now. And unless something is way off on my particular server, ~~0.4 seconds is what PostgreSQL is reporting as the mean (average) time per single comment vote INSERT~~, and post vote INSERT is similar. (NOTE: my server is classic hard drives, 100MB/sec bencharked, not a SSD)

Discussion of the SQL statement for a single comment vote insert: https://lemmy.ml/post/1446775

Every single VOTE is both a HTTP transaction from the remote server and a SQL transaction. I am looking into Postgress supporting batches of inserts to not check all the index constraints at each single insert: https://www.postgresql.org/docs/current/sql-set-constraints.html

Can the Rust code for inserts from federation be reasonably modified to BEGIN TRANSACTION only every 10th comment_like INSERT and then do a COMMIT of all of them at one time? and possibly a timer that if say 15 seconds passes with no new like entries from remote servers, do a COMMIT to flush based a timeout.

Storage I/O writing for votes alone is pretty large...

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[–] phiresky 1 points 1 year ago* (last edited 1 year ago) (2 children)

well tuned a single insert should take less than a tenth of a millisecond on postgresql. even with 10 indexes and a trigger 0.4 seconds is not how long an insert takes. if you're seeing slow times it might be due to very different things, especially the huge locking hot_rank updates that make all inserts / updates on comments,posts table pause until done (which will show up as slower times in the query times you're looking at)

[–] [email protected] 1 points 1 year ago (1 children)

I mixed up the units, but it is taking .4 milliseconds typically. I really want to get some of these stats out of the major servers (Beehaw, Lemmy.world, Lemmy.ml)

[–] phiresky 1 points 1 year ago

yes, real stats would be real helpful. what about those user queries you said take 10 seconds? is that still true? maybe you could publish a new overview over what you find?