Comment by anonymoushn
3 years ago
> So just like any other kind of scheduling?
Yes. Industries that care about latency take some pains to avoid this as well, of course.
> io_uring and epoll have nothing to do with it and can't avoid the problem: the problem is with code that can make progress and doesn't need to poll for anything.
They totally can though? If I write the exact same code that is called out as problematic in the post, my non-preemptive runtime will run a variety of tasks while non-preemptive tokio is claimed to run only one. This is because my `accept` method would either submit an "accept sqe" to io_uring and swap to the runtime or do nothing and swap to the runtime (in the case of a multishot accept). Then the runtime would continue processing all cqes in order received, not *only* the `accept` cqes. The tokio `accept` method and event loop could also avoid starving other tasks if the `accept` method was guaranteed to poll at least some portion of the time and all ready handlers from one poll were guaranteed to be called before polling again.
This sort of design solves the problem for any case of "My task that is performing I/O through my runtime is starving my other tasks." The remaining tasks that can starve other tasks are those that perform I/O by bypassing the runtime and those that spend a long time performing computations with no I/O. The former thing sounds like self-sabotage by the user, but unfortunately the latter thing probably requires the user to spend some effort on designing their program.
> The problem isn't unique to Rust either, and it's going to exist in any cooperative multitasking system: if you rely on tasks to yield by themselves, some won't.
If we leave the obvious defects in our software, we will continue running software with obvious defects in it, yes.
>This sort of design solves the problem for any case of "My task that is performing I/O through my runtime is starving my other tasks."
Yeah, there's your misunderstanding, you've got it backwards. The problem being described occurs when I/O isn't happening because it isn't needed, there isn't a problem when I/O does need to happen.
Think of buffered reading of a file, maybe a small one that fully fits into the buffer, and reading it one byte at a time. Reading the first byte will block and go through epoll/io_uring/kqueue to fill the buffer and other tasks can run, but subsequent calls won't and they can return immediately without ever needing to touch the poller. Or maybe it's waiting on a channel in a loop, but the producer of that channel pushed more content onto it before the consumer was done so no blocking is needed.
You can solve this by never writing tasks that can take "a lot" of time, or "continue", whatever that means, but that's pretty inefficient in its own right. If my theoretical file reading task is explicitly yielding to the runtime on every byte by calling yield(), it is going to be very slow. You're not going to go through io_uring for every single byte of a file individually when running "while next_byte = async_read_next_byte(file) {}" code in any language if you have heap memory available to buffer it.
Reading from a socket, as in the linked post, is an example of not performing I/O? I'm not familiar with tokio so I did not know that it maintained buffers in userspace and filled them before the user called read(), but this is unimportant, it could still have read() yield and return the contents of the buffer.
I assumed that users would issue reads of like megabytes at a time and usually receive less. Does the example of reading from a socket in the blog post presuppose a gigabyte-sized buffer? It sounds like a bigger problem with the program is the per-connection memory overhead in that case.
The proposal is obviously not to yield 1 million times before returning a 1 meg buffer or to call read(2) passing a buffer length of 1, is this trolling? The proposal is also not some imaginary pie-in-the-sky idea; it's currently trading millions of dollars of derivatives daily on a single thread.
You're confusing IO not happening because it's not needed with IO never happening. Just because a method can perform IO doesn't mean it actually does every time you call it. If I call async_read(N) for the next N bytes, that isn't necessarily going to touch the IO driver. If your task can make progress without polling, it doesn't need to poll.
>I'm not familiar with tokio so I did not know that it maintained buffers in userspace
Most async runtimes are going to do buffering on some level, for efficiency if nothing else. It's not strictly required but you've had an unusual experience if you've never seen buffering.
>filled them before the user called read()
Where did you get this idea? Since you seem to be quick to accuse others of it, this does seem like trolling. At the very least it's completely out of nowhere.
>it could still have read() yield and return the contents of the buffer.
If I call a read_one_byte, read_line, or read(N) method and it returns past the end of the requested content that would be a problem.
>I assumed that users would issue reads of like megabytes at a time and usually receive less.
Reading from a channel is the other easy example, if files were hard to follow. The channel read might implemented as a quick atomic check to see if something is available and consume it, only yielding to the runtime if it needs to wait. If a producer on the other end is producing things faster than the consumer can consume them, the consuming task will never yield. You can implement a channel read method that always yields, but again, that'd be slow.
>The proposal is obviously not to yield 1 million times before returning a 1 meg buffer, is this trolling
No, giving a illustrative example is not trolling, even if I kept the numbers simple to make it easy to follow. But your flailing about with the idea of requiring gigabyte sized buffers probably is.
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