Oooo, this looks like an interesting read!
- Participants poll the
GenServer for updates every second.
That seems… inefficient compared to just pushing updates as they happen instead of polling, perhaps with a debouncer? Phoenix makes it easy to push updates to a channel from any process, bypassing the majority of the message passing costs. This is foreboding, lol.
Everything was great - except for one problem: The party kept growing, and thus the number of participants in these events kept growing, too.
And yep, this seems to confirm…
The frequent polling intervals of the first iteration ended up maxing out all eight CPU cores of a
t3a.2xlarge AWS EC2 instance.
And yep, that seems even heavier than expected for just polling on the BEAM, I wonder what other costs were involved…
So I decided to switch from constant polling to a Pub/Sub model. This is also quite easy to do with Elixir and Phoenix: Phoenix comes with its own easy-to-use PubSub module.
Yay! Hopefully straight to the socket processes and not re-rendering with LiveView (which does it so incredibly inefficiently compared to some other thing libraries).
A three-day convention packed with votes and almost 3,000 eligible members in Germany.
Didn’t stress test it first?!? Still though, 3k doesn’t sound like much, I’ve stressed drab at work to over 40k on a single core without issues.
It was like watching a trainwreck: As soon as the server was up again, RAM usage immediately started climbing, and climbing … until the inevitable out-of-memory crash.
Oooo I can see so many possible causes…
The LiveView controller process would then receive these messages, set the
@participants assign and render an updated view:
…oh wow, right, LiveView stores the changes inside each liveview process instead of shared data or just pushing it to the client to handle like you can in Drab (I still say Drab is overall better designed than LiveView, trivial not to cause this kind of issue in it, where LiveView encourages these issues…)…
With dozens of these updates happening per second as participants were joining the convention, messages were piling up in the inbox of the LiveView admin controller processes faster than they could be handled.
Eh, I wouldn’t think so, when a process on the beam sends a message to another process on the beam on the same system it has backpressure, so if the mailbox grows then the sender process gets scheduled less and less often until it practically is paused… Though if PubSub were used to talk to intermediary processes I could see issues…
My laptop crashed, the theory had been confirmed!
- Why on earth would the laptop crash from a single process consuming excess memory?!? What on earth was the OS being used?!
- No, I still think it was something else than the mailbox… Like using liveview re-rendering huge swaths of things instead of a better Drab-like model of pushing updates to the client to handle. Still should have debounced the changed data, which Drab would have automatically done by just broadcasting straight to the clients from the change process instead of an intermediary process with its own memory and mailbox and stack and all.
I then wanted the LiveView process to occasionally check if this other assign had been modified and, if so, also update
More polling? Why not a timeout message when a change comes in? Or better yet broadcast straight to the clients instead of going through intermediary processes per client (that sounds so heavy for shared data…).
With thousands of updates coming in at the same time, neither Firefox nor Chromium stood a chance.
Debouncing and batching!
I implemented a mechanism to do so at most once every second.
Close enough to debouncing, though more costly when no updates are happening, lol.
- Avoid large payloads in
Phoenix.PubSub if possible
Yep, best to send only changes, and let the pubsub go straight to the client socket process to be handled on the client instead of intermediary re-rendering processes.
- Throttle PubSub events at the sender level to avoid clogged process inboxes
Yeah, pubsub doesn’t backpressure as much as one would hope, this is why sending directly to the socket processes would be far better (which use pubsub internally anyway, still debounce your data!).
assign/3 in LiveView always causes an update via Websocket, even if no changes were made