alt Hacker NewsWith packages like this (lots of cores, multi-chip packaging, lots of memory channels), the architecture is increasingly a small cluster on a package rather than a monolithic CPU.
I wonder whether the next bottleneck becomes software scheduling rather than silicon - OS/runtimes weren’t really designed with hundreds of cores and complex interconnect topologies in mind.
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I don't think there are any fundamental bottlenecks here. There's more scheduling overhead when you have a hundred processes on a single core than if you have a hundred processes on one hundred cores.
The bottlenecks are pretty much hardware-related - thermal, power, memory and other I/O. Because of this, you presumably never get true "288 core" performance out of this - as in, it's not going to mine Bitcoin 288 as fast as a single core. Instead, you have less context-switching overhead with 288 tasks that need to do stuff intermittently, which is how most hardware ends up being used anyway.
I think linux can handle upto 1024 cores just fine.
That's a great point. Linux has introduced io_uring, and I believe that gives us the native primitives to hide latency better?
But that's just one piece of the puzzle, I guess.
There definitely are bottlenecks. The one I always think of is the kernel's networking stack. There's no sense in using the kernel TCP stack when you have hundreds of independent workloads. That doesn't make any more sense than it would have made 20 years ago to have an external TCP appliance at the top of your rack. Userspace protocol stacks win.
> OS/runtimes weren’t really designed with hundreds of cores and complex interconnect topologies in mind.
I mean....
IMO Erlang/Elixir is a not-terrible benchmark for how things should work in that state... Hell while not a runtime I'd argue Akka/Pekko on JVM Akka.Net on the .NET side would be able to do some good with it...[0] Similar for Go and channels (at least hypothetically...)
[0] - Of course, you can write good scaling code on JVM or CLR without these, but they at least give some decent guardrails for getting a good bit of the Erlang 'progress guaranteed' sauce.
> I wonder whether the next bottleneck becomes software scheduling rather than silicon
Yep, the scheduling has been a problem for a while. There was an amazing article few years ago about how the Linux kernel was accidentally hardcoded to 8 cores, you can probably google and find it.
IMO the most interesting problem right now is the cache, you get a cache miss every time a task is moving core. Problem, with thousands of threads switching between hundreds of cores every few milliseconds, we're dangerously approaching the point where all the time is spent trashing and reloading the CPU cache.
Yes there are scheduling issues, Numa problems , etc caused by the cluster in a box form factor.
We had a massive performance issue a few years ago that we fixed by mapping our processes to the numa zones topology . The default design of our software would otherwise effectively route all memory accesses to the same numa zone and performance went down the drain.