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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GREAT NEWS about Lemmy Server Performance, another major SQL mistake has been discovered today: every single comment & post create (INSERT) is updating ~1700 rows in the site_aggregates table (lemmy.ml)

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

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Details here: https://github.com/LemmyNet/lemmy/issues/3165

This will VASTLY decrease the server load of I/O for PostgreSQL, as this mistaken code is doing writes of ~1700 rows (each known Lemmy instance in the database) on every single comment & post creation. This creates record-locking issues given it is writes, which are harsh on the system. Once this is fixed, some site operators will be able to downgrade their hardware! ;)

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[–] [email protected] 29 points 1 year ago* (last edited 1 year ago) (1 children)

It's not on every comment, it's mostly triggered on deletions and edits. The problem is actually infintesimally worse and 1700 rows are updated if you delete 3 comments. If you delete more it's exponential and just straight up fails and locks your database.

I'll probably put a patch in there later tonight and then see about a PR unless someone else does.

[–] [email protected] 28 points 1 year ago* (last edited 1 year ago) (1 children)

It’s not on every comment,

My testing with latest code is that it is indeed on every single comment INSERT, every new comment. I have the ability to view my live data while using Lemmy: https://lemmyadmin.bulletintree.com/query/raw_site_aggregates?output=table

Every one of the 1486 rows on that table gets +1 on comment when I post a new comment on my instance.

it’s mostly triggered on deletions and edits

That is not correct. Edits do not change the count of comments column on site_aggregates - because the number isn't changing. Deletes (of a comment or post) in Lemmy are also not SQL DELETE statements, they are just a delete data column in the table. That DELETE PostgreSQL trigger only gets run when a end-user cancels their Lemmy account in their profile.

[–] [email protected] 11 points 1 year ago

Ah Gotcha. That's true, but the cascading issue that causes thousands of inserts happens on a delete.

That table update you're looking at is blazing fast due to the relatively low number of sites, until you run it thousands of times on a delete.