this post was submitted on 04 Jul 2023
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Lemmy has a memory leak? Or, should I say, a "lemmory leak"?
A pretty bad one at that...
But... but... Rust...
Rust makes holes and that's how leaks happen
Underrated comment
Rust protects you from segfaulting and trying to access deallocated memory, but doesn't protect you from just deciding to keep everything in memory. That's a design choice. The original developers probably didn't expect such a deluge of users.
Leaking memory is safe
I’m calling it - if there’s actually a memory leak in the Rust code, it’s gonna be the in memory queues because the DB’s iops can’t cope with the number of users.
I think I found what eats the memory. DB iops isn't the cause - looks like the server doesn't reply before all the database operations are done. The problem is the unbounded queue in the activitypub_federation crate, spawned when creating the ActivityQueue struct. The point is, this queue holds all the "activities" - events to be sent to federated instances. If, for whatever reason, the events aren't delivered to all the federated servers, they are retried with an exponential backoff for up to 2.5 days. If even a single federated instance is unreachable, all events remain in memory. For a large instance, this will eat up the memory for every upvote/downvote, post or comment.
Lemmy needs to figure out a scalable eventual consistency algorithm. Most importantly, to store the messages in the DB, not in memory.
You should consider bringing this up at !lemmyperformance@lemmy.ml
You should take this entire comment and paste it in as a issue on the Lemmy Github Issues page.
I think the devs have been aware of the issue, theoretically, for a while. A proper solution requires some significant changes, so it was being postponed because this wasn't considered urgent.
A lemmory meek, obviously!
On some of the latest release candidates I think so.
Wait isn't lemmy written in rust how do you create a memory leak in rust? Unsafe mode?
That's not a memory leak though. That's just hording memory. Leaked memory is inaccessible.
In the example it’s inaccessible.
It's not. The vec is still accessible and if it goes out of scope rust automatically clears the memory.
But it will never fall out of scope because of the loop
Exactly but it's still accessible since it's in scope.
Right but where does it say that something has to be inaccessible to qualify as a memory leak?
"A memory leak is a process in which a program or application persistently retains a computer’s primary memory. It occurs when the resident memory program does not return or release allocated memory space, even after execution, resulting in slower or unresponsive system behavior." Source
That’s not a serious source. Any unbound allocation is a memory leak if it serves no useful purpose.
Exactly.