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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[–] [email protected] 2 points 1 year ago (2 children)

Batching the inserts up only kicks the can down the road a few weeks. We need a 500x improvement in insertion time.

[–] [email protected] 2 points 1 year ago (1 children)

The proposal has been raised (by me) to move all federation out of lemmy_server into a different service and have a queue in there. I think that opens up to people working and updating the code better. The email systems I have worked with that have a database storage backend have used their own MTA service, not run in the main app's core. I also think Reddit does data acceptance before it gets to PostgreSQL too - as I've seen comments get backed up when one of their servers or services goes offline.

[–] phiresky 1 points 1 year ago

it is already in a different repo, just running from the same process. since 0.18 (with debug mode off) it should also be running in a somewhat efficient multi-thread environment