this post was submitted on 15 Jun 2023
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My first experience with Lemmy was thinking that the UI was beautiful, and lemmy.ml (the first instance I looked at) was asking people not to join because they already had 1500 users and were struggling to scale.

1500 users just doesn't seem like much, it seems like the type of load you could handle with a Raspberry Pi in a dusty corner.

Are the Lemmy servers struggling to scale because of the federation process / protocols?

Maybe I underestimate how much compute goes into hosting user generated content? Users generate very little text, but uploading pictures takes more space. Users are generating millions of bytes of content and it's overloading computers that can handle billions of bytes with ease, what happened? Am I missing something here?

Or maybe the code is just inefficient?

Which brings me to the title's question: Does Lemmy benefit from using Rust? None of the problems I can imagine are related to code execution speed.

If the federation process and protocols are inefficient, then everything is being built on sand. Popular protocols are hard to change. How often does the HTTP protocol change? Never. The language used for the code doesn't matter in this case.

If the code is just inefficient, well, inefficient Rust is probably slower than efficient Python or JavaScript. Could the complexity of Rust have pushed the devs towards a simpler but less efficient solution that ends up being slower than garbage collected languages? I'm sure this has happened before, but I don't know anything about the Lemmy code.

Or, again, maybe I'm just underestimating the amount of compute required to support 1500 users sharing a little bit of text and a few images?

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[–] [email protected] 87 points 1 year ago (10 children)

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.

[–] [email protected] 59 points 1 year ago (1 children)

Yup was just typing a comment to basically this effect. Federation adds a ton of overhead -- you can still do things fairly efficiently, but every interaction having to fan out to (and fan in from!) many servers instead of like a single RDBMS is gonna cost you.

In all likelihood the code is not as efficient as it could be, but usually you get time to work those out gradually. A giant influx of users quickly turns "TODO: fix in the next six months" into "Oh god the servers are melting fuck fuck."

That said, assuming the devs can get over this hump, I suspect using a compiled language will pay off long-term. Sure things will still be primarily IO-bound, but making things less CPU-bound is usually a good thing.

For some illustrative examples: Mastodon is in Ruby and hits dumb scaling limitations far more often than other fedi microblogs. Pleroma/Akkoma are Elixir (and BEAM is super well optimized for fast message passing/scaling/IO), Calckey (primarily Typescript) is moving some code to Rust, GoToSocial (Golang) is able to run in a fraction of the resources of Mastodon. The admins of one of the bigger tech instances recently announced they're basically giving up on administrating Mastodon and are instead going to write a new server from scratch in a compiled language because it's easier for them than scaling a Rails monolith.

TL;DR everything is IO-bound til it's not.

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[–] [email protected] 19 points 1 year ago (2 children)

I'm pretty sure the fediverse needs a new kind of node at some point. If we assume, that almost every larger instance is connected to almost every other larger instance directly, then there's a ton of duplicated and very small messages.

There needs to be some kind of hub in-between to aggregate and route this avalanche. Especially if, like you wrote, every upvote is a message, the overhead (I/O, unmarshalling, etc) is huge.

[–] [email protected] 8 points 1 year ago* (last edited 1 year ago) (2 children)

You mean like centralizing the fediverse? Who hosts the hub? Who maintains it? In which country? Who pays for it?

[–] [email protected] 9 points 1 year ago (4 children)

Not a single hub, multiple ones.

Anyone can host a hub, federated instances can negotiate the intersection of hubs they both trust and then send traffic that way. That could mean, a single comment might be sent to, say, five hubs and each hub then forwards to 50 instances or so.

Since the hubs are rather simple, they can scale very easily and via cryptographic ratchets, all instances can make sure, they received the correct messages.

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[–] [email protected] 12 points 1 year ago

Well said. Thanks for sharing your experience and those insightful links.

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[–] [email protected] 74 points 1 year ago (3 children)

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.

[–] [email protected] 13 points 1 year ago (1 children)

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.

You mean like coalescing multiple events into a single message, or…? (I don't know anything about ActivityPub, so apologies if this is a stupid question!)

[–] [email protected] 11 points 1 year ago (1 children)

correct. I've been looking for the thread to try and find it for you, but haven't been having any luck. People have been discussing exactly that though, but it seems like it could cause some problems with vote faking. Anyway, it is being worked on!

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[–] [email protected] 66 points 1 year ago (1 children)

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

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[–] [email protected] 49 points 1 year ago (14 children)

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.

[–] [email protected] 25 points 1 year ago* (last edited 1 year ago) (1 children)

Not relevant to lemmy (yet), but this does break down a bit at very large scales. (Source: am infra eng at YouTube.)

System architecture (particularly storage) is certainly by far the largest contributor to web performance, but the language of choice and its execution environment can matter. It's not so important when it's the difference between using 51% and 50% of some server's CPU or serving requests in 101 vs 100 ms, but when it's the difference between running 5100 and 5000 servers or blocking threads for 101 vs 100 CPU-hours per second, you'll feel it.

Languages also build up cultures and ecosystems surrounding them that can lend themselves to certain architectural decisions that might not be beneficial. I think one of the major reasons they migrated the YouTube backend from Python to C++ isn't really anything to do with the core languages themselves, but the fact that existing C++ libraries tend to be way more optimized than their Python equivalents, so we wouldn't have to invest as much in developing more efficient libraries.

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

It is fairly relevant to lemmy as is. Quite a few instances have ram constraints and are hitting swap. Consider how much worse it would be in python.

Currently most of the issues are architectural and can be fixed with tweaking how certain things are done (i.e., image hosting on an object store instead of locally).

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[–] [email protected] 44 points 1 year ago (3 children)

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.

[–] balder1991 10 points 1 year ago* (last edited 1 year ago)

I mean, comparing it to Python is kinda unfair as Rust is closer to C and Python is one of the slowest languages out there. There is a whole spectrum of languages between Rust and Python. The final reason was probably that the devs were comfortable enough with Rust.

[–] [email protected] 8 points 1 year ago

I would like to point out that this article from Discord could be considered outdated these days. This comparison is using a old version of Go, that in recent years got a optimization on the GC and now would have much lower spikes than those showed in the article

Still, Rust surely will give you a higher performance and lower latency than Python

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[–] [email protected] 42 points 1 year ago (1 children)

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.

[–] [email protected] 16 points 1 year ago* (last edited 1 year ago) (2 children)

Which is fine. If they wanted to learn Rust and wrote inefficient code, good for them. I appreciate their efforts. Rust can certainly be beaten into shape and perform well enough in the end.

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[–] sznio 26 points 1 year ago (4 children)

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.

[–] [email protected] 20 points 1 year ago (3 children)

I maintain and host ntfy.sh, an open source push notification service. I have a constant 9-12k WebSocket and HTTP stream connections going, and I host it on a two core machine with an average load average of less than 1. So I can happily tell you that it's not WebSockets. Hehe.

My money would be on the federation. Having to blast/copy every single comment to every single connected instance seems like a lot.

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[–] [email protected] 7 points 1 year ago (1 children)

They're gonna move away from we sockets within a couple of weeks, from what I hear

[–] [email protected] 8 points 1 year ago

That's a good move IMHO. Honestly I don't want my UI to randomly shift down when new messages come in from syncing with another instance.

The right move would be to make a page that renders once and then only updates when you refresh the page. And then use web push for message notifications.

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[–] dragontamer 21 points 1 year ago (2 children)

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

[–] [email protected] 14 points 1 year ago (1 children)

The lemmy.ml post that asks people to go elsewhere is still stickied and the first thing it mentions is that they have upgraded the server. So that does suggest that compute scaling is an issue.

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[–] [email protected] 18 points 1 year ago (2 children)

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.

[–] [email protected] 12 points 1 year ago* (last edited 1 year ago) (1 children)

Rust is not to blame, but that code that has been written in Rust might be to blame.

The algorithms used have more effect than the language used, and Rust might make using certain algorithms more painful and thus steer programmers towards less efficient algorithms. Using clone is often an example of this, it's a little easier and gets around some borrow checker difficulties. (This is true in general, but I don't know if this is what has happened with Lemmy.)

Look at salvo [diesel] coming it at #200+ on this benchmark1, lots of programming languages have at least one framework that is faster on the microbenchmark. This isn't especially meaningful, but it does show that, let's say, a feature rich framework in Rust might end up being slower than a Python framework that's laser focused on the specific use case.

[–] [email protected] 7 points 1 year ago (2 children)

There’s a catch here, something I read someone mention on Hacker News and I agree. Python is easy when you don’t care about performance; the moment you need to worry about it, all the easiness gets thrown away.

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[–] [email protected] 7 points 1 year ago

What makes you think that inefficient rust is slower than efficient Python?

Well of course it can be. Performance issues are very often algorithmic rather than from execution efficiency.

Rust won't make it slow for sure, but it's not enough to ensure it's fast.

[–] [email protected] 12 points 1 year ago

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."

[–] TeeTwoLee 12 points 1 year ago

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.

[–] [email protected] 11 points 1 year ago (1 children)

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

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[–] [email protected] 9 points 1 year ago

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.

[–] [email protected] 9 points 1 year ago (2 children)

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!

[–] [email protected] 8 points 1 year ago (10 children)

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!

No it wouldn't, that's called the N+1 query problem and it can be avoided by writing more efficient queries

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[–] [email protected] 7 points 1 year ago

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!

I mean, I hope that surely can't be the case. There is no reason why they wouldn't get all of those comments in a single request even if it were separate rows in the db. I initially thought that they'd have to query each individual instance participating in a thread but even that isn't the case because the fediverse protocol makes each instance mirror the content from others, meaning that your local instance should already have the necessary data in one place, making it easy to load it to the users.

[–] [email protected] 7 points 1 year ago* (last edited 1 year ago)

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.

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