Programming

The numbers are a little higher than you mention (currently ~3.2k active users). The server isn't very powerful either, it's now running on a dedicated server with 6 cores/12 threads and 32 gb ram. Other public instances are using larger servers, such as lemmy.world running on a AMD EPYC 7502P 32 Cores “Rome” CPU and 128GB RAM or sh.itjust.works running on 24 cores and 64GB of RAM. Without running one of these larger instances, I cannot tell what the bottleneck is.

The issues I've heard with federation are currently how ActivityPub is implemented, and possibly the fact all upvotes are federated individually. This means every upvote causes a federation queue to be built, and with a ton of users this would pile up fast. Multiply this by all the instances an instance is connected to and you have an exponential increase in requests. ActivityPub is the same protocol used by other federated servers, including Mastodon which had growing pains but appears to be running large instances smoothly now.

Other than that, websockets seem to be a big issue, but is being resolved in 0.18. It also appears every connected user gets all the information being federated, which is the cause for the spam of posts being prepended to the top of the feed. I wouldn't be surprised if people are already botting content scrapers/posters as well, which might cause a flood of new content which has to get federated which causes queues to back up; this is mostly speculation though.

As it goes with development, generally you focus on feature sets first. Optimization comes once you reach a point a code-freeze makes sense, then you can work on speeding things up without new features breaking stuff. This might be an issue for new users temporarily, but this project wasn't expecting a sudden increase in demand. This is a great way to show where inefficiencies are and improve performance is though. I have no doubt these will be resolved in a timely manner.

My personal node seems to use minimal resources, not having even registered compared to my other services. Looking at the process manager the postgres/lemmy backend/frontend use ~250MB of RAM.

For now, staying off lemmy.ml and moving communities to other instances is probably best. The use case of large instances anywhere near the scale of reddit wasn't the goal of the project until reddit users sought alternatives. We can't expect to show up here and demand it work how we want without a little patience and contributing.

Hi, programming.dev owner here. From what I've been seeing it's a lot of memory issues. We were hitting swap which was causing massive disk io. You can see what happened with the disk io immediately after the upgrade to more memory. I know at least one reason is being resolved in this PR

We were also having issues with the nginx config. There were some really weird settings that I don't think were necessary. Finally, the federation is quite busy. So if someone subscribes to events from 10 different servers, we pull in every single event, even upvotes. There's currently a lot of work being done around this stuff.

I don't think Rust is the problem. I think it's just a growth thing. Every platform has growth challenges, things grow in ways that you never expect. You might have thought that it was going to be IO constrained due to the federation, but in reality it's memory constrained because memory is actually the most expensive thing to have on a server. etc.

It could be the devs just like programming in Rust. It's a nice language lol

I would say that it's extremely unlikely.

Websites in general are never limited by raw code execution, they are mostly limited by IO. Be that disk IO as files are read and written, database IO as you need to execute complex queries to gather all the data to build the user timeline, and network IO to transfer data to and from the user. For decentralized social media like Kbin or Lemmy its even more IO limited as each instance needs to go back and forth to other instances to keep up-to-date data.

Websites usually benefit much more from caching and in-memory databases to keep frequently used data in fast storage.

This is why simple, high level, object oriented, garbage collected languages have become so common. All the CPU performance penalties they incur don't actually affect the website performance.

One benefit of using rust for webservers in general is that it's possible to have a consistently lower latency compared to GCd langues: discord mentions this in their article about migrating to rust from go: https://discord.com/blog/why-discord-is-switching-from-go-to-rust

Another difference between rust and e.g. python is that rust expects you to invest more time to get code that's runnable in the first place, but likely more well optimized and correct.

In my experience from writing rust, the language pushes you to write more efficient code compared to python because it makes things like copying visible and also because it's easier to reason about memory usage compared to garbage collection which means that you're more likely to have a useful model of the performance cost of various things while you're programming.

It's possible that a hypothetical lemmy written in python would have allowed the devs to do some big picture optimizations that they haven't had the time to do yet in the rust version, so for the time being it might be slower than a python alternative.

Rust is likely to catch up though: eventually the rust version will probably also have this optimization while the python version has to resort to make smaller optimizations that the rust version already had in the first version of the code or that you get for free from the language.

I think the devs openly stated they aren't backend bods and asked for help optimising the database as a priority. There's a bit of work going on on github to sort that out I think. Anyone reading this who can optimise postgresql or contribute to a database agnostic retool should probably speak to the devs as I imagine you'd be welcome.

I wish I could help so much but I doubt they're going to retool into .net haha.

Pulling this out of my ass, but I think the problem might be in Lemmy using websockets.

I feel like supporting 1500 simultaneous users making a request every 10-20 seconds is easier than keeping 1500 websockets alive.

Irregardless, Lemmy does feel very snappy compared to other websites I've had the displeasure of using. Main problem is low robustness in the RPC layer.

What makes you think that inefficient rust is slower than efficient Python? I mean, sure it could be possible if you are actively trying to make rust slow, but rust is multiple order of magnitude faster than python. If rust was to blame here then I don't think any language could be fast enough.

Its not code that seems to have difficulty scaling, it seems to be user curation and user trust that is being difficult to scale.

When it comes to architecture I'm not a genius, but I think a lot of how federation works is tough.

Plus, even if you have a perfectly efficient backend it can only handle so much based on the hardware you're running on. "Struggling to scale" can just mean "struggling to afford better hardware."

I think threads like this one: https://news.ycombinator.com/item?id=28440742 help explain the complexity of performance. There's considerations from the development process, existing technology, simplicity, and more.

At this point I think the bottleneck is largely that every server has needed to upgrade for this wave of the reddit migration.

Whatever the performance bottlenecks are, I hope they can get them sorted so that any exponential growth caused by Reddit at the end of the month can be capitalized on.

I've got a feeling that other redditors who want to flee / branch out will look at these sites and so long as they see posts, comments and that it has a decent UI and loads well then people will stick around and build communities here.

I agree, hearing about scaling issues so early into adoption is concerning. Lemmy advocates say "horizontal scaling is already built in! just add more instances!", but that doesn't explain the problem.

It's all just text! By my guess too, handling text alone a server should easily support a thousand concurrent users, and hundreds of thousands of daily users. A RasPI should handle thousands. I've heard the bottleneck is the database? In that case Rust is not to blame, Postgres is.

But my fear is that the data structures are implemented in a trivial way. If you have a good reddit-sized thread with a thousand comments, but you store each comment as a separate database entry, then every pageview will trigger a thousand database lookups! The way I imagined making a reddit clone is that I would store the comments as a flat list with some tree data on top, such that serving a single page with 1000 comments is no different that streaming a 100K text file. I'll go take a look how Lemmy does it currently once I get the courage!

I'm not convinced using rust is all that useful for web development. There are already plenty of other mature well optimized solutions for backend web development that includes a lot of security, qol features ootb like mature orm's with optimized database access which regardless of how fast your code is can bottleneck your site much faster if you don't have smart database access.

Yeah, theoretically rust can be faster than ruby/python/node, but it's harder to optimize it enough to get to that point and you will have a much harder time finding devs to work on such a project because the amount of backend devs that have enough rust experience is so small, they're like on the opposite sides of the ven diagram of languages for web developers.

On top of all that, languages which are heavily used for web development often get low level optimizations baked into the frameworks/languages so you can get pretty amazing performance uplifts over time to bring it to the level of lower level languages.

The real benefit as I see it for using rust for backends is memory safety.