ERL-1079: High CPU usage with "+sbwtdio none"

[OTP-Maintainer]

Original reporter: huiliu
Affected version: OTP-22.0
Fixed in version: OTP-22.1.8
Component: erts
Migrated from: https://bugs.erlang.org/browse/ERL-1079

---

We are working on a I/O heavy application. Previously with OTP 21.2(erts-10.2), we disable busy waiting of dirty I/O scheduler by "+sbwtdio none". Recently we upgraded to OTP22.07(erts-10.4.4), we see way too much CPU usage with same "+sbwtdio none". 

I did some CPU profiling by Linux perf. It seems wake_dirty_scheduler in erl_process.c is the one who consumes most of cpu cycles. It is busy with a spinlock. Is that a known issue in OTP 22 that "+sbwtdio none" is broken?

BTW - if I switch to "+sbwtdio short"(default option), CPU usage is back to normal level.   

[OTP-Maintainer]

rickard said:

Spinlocks are notoriously bad when they get highly contended. I pushed a branch {{rickard/dirty-sleepers-contention}} (based on OTP-22.1.7) to my [github repo|https://github.com/rickard-green/otp] which replace the usage of spinlocks with mutexes for this. The contention is still there, but it should behave much better. Please try and get back with the result.

I really would like to rewrite how dirty schedulers sleep and are woken, but haven't got time for that right now. Have put it on the todo-list though.