yield_now not yielding since MAX_TASKS_PER_TICK in LocalSet is hardcoded

[arielb1]

Version

Tokio 1.21.2

Platform

Linux 5.15.0-53-generic #59-Ubuntu SMP Mon Oct 17 18:53:30 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Description

The MAX_TICKS_PER_TASK parameter in LocalSet is hardcoded to be 61.

This means that if, while a task is waiting for IO, there is a task in a LocalSet that is performing

loop {
    expensive_computation();
    tokio::task::yield_now().await;
}

The expensive_computation will run up to 61 times before IO is polled, which will cause high tail latencies.

cc @carllerche

[arielb1]

I would also like some way to have a version of yield_now that makes the task not wake up until IO is polled.

Currently I do some evil hack that creates a pipe and waits for the pipe to be readable, but I hate that.

[Darksonn]

In FuturesUnordered, we make it always yield if the task wakes itself. That could be one approach to this.

[carllerche]

I believe #5223 should resolve your issue here. While it would still be nice for LocalSet to be configurable, yielding no longer triggers this.