[captainmuon]

One thing that really goes against my intuition is that user space threads (lightweight treads, goroutines) are faster than kernel threads. Without knowing too much assembly, I would assume any modern processor would make a context switch a one instruction affair. Interrupt -> small scheduler code picks the thread to run -> LOAD THREAD instruction and the processor swaps in all the registers and the instruction pointer.

You probably can't beat that in user space, especially if you want to preempt threads yourself. You'd have to check after every step, or profile your own process or something like that. And indeed, Go's scheduler is cooperative.

But then, why can't you get the performance of Goroutines with OS threads? Is it just because of legacy issues? Or does it only work with cooperative threading, which requires language support?

One thing I'm missing from that article is how the cooperativeness is implemented. I think in Go (and in Java's Project Loom), you have "normal code", but then deep down in network and IO functions, you have magic "yield" instructions. So all the layers above can pretend they are running on regular threads, and you avoid the "colored function problem", but you get runtime behavior similar to coroutines. Which only works if really every blocking IO is modified to include yielding behavior. If you call a blocking OS function, I assume something bad will happen.

[masklinn]

> And indeed, Go's scheduler is cooperative.

It hasn't been cooperative for a few versions now, the scheduler became preemptive in 1.14. And before that there were yield points at every function prolog (as well as all IO primitives) so there were relatively few situations where cooperation was necessary.

> Without knowing too much assembly, I would assume any modern processor would make a context switch a one instruction affair.

Any context switch (to the kernel) is expensive, and way more than a single operation. The kernel also has a ton of stuff to do, it's not just "picks the thread to run", you have to restore the ip and sp, but also may have to restore FPU/SSE/AVX state (AVX512 is over 2KB of state), traps state.

Kernel-level context switching costs on the order of 10x what userland context switching does: https://eli.thegreenplace.net/2018/measuring-context-switchi...

> LOAD THREAD

There is no load thread instruction

[jaytaylor]

> It hasn't been cooperative for a few versions now, the scheduler became preemptive in 1.14. And before that there were yield points at every function prolog (as well as all IO primitives) so there were relatively few situations where cooperation was necessary.

Since co-op was most unnecessary, do you know why it was changed to preemptive or what the specific cases were that are resolved with preemptive scheduling?

[mseepgood]

Tight loops without function calls.

[kgeist]

IIRC in earlier versions, an infinite loop without function calls could freeze the entire runtime: GC's stop the world event is triggered => goroutines are being suspended cooperatively => but this one goroutine never enters a function prolog and thus never suspends => all goroutines except one are suspended and there's no progress. Preemptive scheduling is much more robust. Although it's solvable in cooperative scheduling with an additional suspension check at the end of each loop, but it adds overhead for all loops. If I remember correctly, .NET or JVM implement safe points for GC (which can be used to switch contexts cooperatively as well) by simply reading a pointer from a special preallocated virtual memory page which is remapped to nothing when a stop-the-world event is triggered, so such a read traps into an invalid memory handler where you can park your thread. But I'm not sure how costly it is for thousands/millions of coroutines.

[jesboat]

> But I'm not sure how costly it is for thousands/millions of coroutines.

Still cheap: you only need to preempt the threads which are actively running user code. If a coroutine is ready to run, but not actually running, you don't have to do anything with it (as long as you check for safepoints before entering user code.) That means your safepoints cost is `O(os threads currently running user code)` which in most runtimes is `O(num cores)`

[throwaway894345]

Replace “tight” with “long-running/infinite” but yeah, otherwise this is correct.

[pron]

There are a few reasons why context switching in user mode could be faster, but that has little if anything to do with the performance benefits of usermode threads. The performance benefit of usermode threads is a result of their quantity, due to Little's law. They're not "faster", just more numerous, and that's what you need for higher throughput. More precisely, OS threads, because of their scarcity, introduce an artificial bound on throughput that's lower than what the hardware can support, and usermode threads remove that bound.

More here: https://inside.java/2020/08/07/loom-performance/

As to why it's hard for the OS to allow that many threads, the OS would need to keep thread stacks small and resizable, and that is hard to do if you don't know the specifics of how the language uses the stack. For example, to accommodate low-level languages that allow pointers into the stack you would need to manipulate virtual memory (to keep addresses valid), but that only works at a page granularity, or to introduce split stacks, which would require a new kind of ABI known to compilers (and would probably have a cost to performance).

[ibraheemdev]

> More precisely, OS threads, because of their scarcity, introduce an artificial bound on throughput that's lower than what the hardware can support, and usermode threads remove that bound.

Why are OS threads scarce? The OS allocates thread stacks lazily. Given a kernel stack of ~8kb (two pages) and a user stack of ~4kb, one could spawn 100k threads with just over 1GB. A userspace runtime will allow you to bring that number down, but if you're at the scale of concurrency it is unlikely to matter much.

[duped]

> I would assume any modern processor would make a context switch a one instruction affair. Interrupt -> small scheduler code picks the thread to run -> LOAD THREAD instruction and the processor swaps in all the registers and the instruction pointer.

It can't be a single instruction, since the details of what a "context" contains depends on the OS and ABI. For example on Linux, the signal mask is a part of the OS thread context (but usually not user thread contexts) which requires a syscall to retrieve it from kernel memory before saving it in the context.

The reason why user threads are so much faster than OS threads is precisely because it can be reduced to a handful of instructions without caring about all the details that OS threads need to care about.

> Which only works if really every blocking IO is modified to include yielding behavior. If you call a blocking OS function, I assume something bad will happen.

That's exactly what Go does, they introduce yield points into function prologues and i/o ops. You don't have direct FFI calls in Go so it's not as big of an issue. It's roughly the same problem as GC safepoints in multithreaded interpreters that support FFI.

[YZF]

There is a ton of context associated with OS/kernel threads. Virtual memory, security, I/O. While there is some hardware acceleration for those in modern processors there isn't anything like LOAD THREAD and even with CPU support it's still very expensive.

You get an interrupt, then the kernel needs to load its own context (the tables it needs to access), then the kernel needs to do the expensive switch.

In user space you have a lot less context. The actual switching is pretty much the cost of a function call. If you need preemption that's a different story and mostly depends on what facilities are available for that. Inserting preemption checks is a little hacky (hello Go ;) ) but what can you do.

EDIT: It's worthwhile noting there's indirect costs like caches being stale. Lightweight/green threads will often work on shared data structures so the caches are more likely to have something useful in them. They may even share the code.

[garaetjjte]

One of the issues is that OS schedulers are complex, and actually much more expensive than context switch itself. You can mitigate this with user-mode scheduling of kernel threads: https://www.youtube.com/watch?v=KXuZi9aeGTw

[gpderetta]

Just the interrupt itself is going to cost a couple of order of magnitude more than the whole userspace context switch.

[heavyset_go]

I don't have any specific recommendations to give you, but skim through an operating systems text book, or college course that puts its slides and whatnot online, when it comes to kernel context switching. It'll give you an idea of what kind of work a kernel must do when context switching between threads and processes, and why userspace multitasking can be more efficient.

[asien]

>I would assume any modern processor would make a context switch a one instruction affair.

Has been the historic assumption, has been proven to be wrong by every possible benchmark.

Consider tech empower[0] for raw stack performance , runtime level threads outperform IO threads since OS thread were designed to be mapped on physicals cores.

This is very expensive and inefficient.

Creating one thread for every request you have ( Apache + PHP ) will exhaust the hardware after a few thousands/qps target.

Runtime can indeed have millions of those “lightweight threads” without killing your machine since they create a pool from physical threads and tap into IO events to efficiently switch or resume contexts. This is by far much faster.

[0] https://www.techempower.com/benchmarks/#section=data-r20&hw=...

[astrange]

> Creating one thread for every request you have ( Apache + PHP ) will exhaust the hardware after a few thousands/qps target.

PHP installations more realistically use nginx and FastCGI. This is not one thread per request and it’s also a better design than hosting your entire server and every user request in the same process; that’s just asking for security issues.

[seunosewa]

On Linux, 99% of the time, you want kernel threads. They can be configured to use very little stack memory. They can be configured to reduce the scheduling overhead by using NOOP scheduling which works for certain workloads. They are rock solid.

Spawning millions of unnecessary user-space threads because they are supposed to be more efficient than kernel threads is rarely the best solution to any problem imho.

[danachow]

> modern processor would make a context switch a one instruction affair.

Reduced instruction set (complexity) is a hallmark of modern processor designs, not the other way around.

You might want to read about what is involved in a task switch (either "thread" with same memory mapping, or "process") but it is not something that is conducive to reasonably carry out in one instruction.