RFC: AArch64: Implementing spin pauses with ISB

[Andrew Haley]

On 8/17/21 9:42 PM, Astigeevich, Evgeny wrote:
> 
>> The ISB instruction wasn't intended to be used for that purpose...
> 
> It might be a time for YIELD to be a real instruction, especially on Neoverse. High thread contention is a typical situation in server workloads.
> If it would be great if Neoverse architects consider this.

Maybe, but recent experience from Intel (where the delay was changed from 20-30
clocks to 200) causing regressions in some areas, suggests it's very problematic.

>> Your experiments were with one ISB - did you experiment at all with multiple ISBs? I'm curious as to what the overall effect would be.
> 
> Yes, there were experiments with 2 ISBs. With 2 ISBs the performance improvements were less. Graviton 2 performance engineers' explanation of this is that spins should target 15-30ns. One ISB allows to be within these limits. Two and more ISBs get longer spins. It increases chances of an expensive code path and the OS to reschedule threads.

How application dependent is this? Does it depend on how many threads are
contending for a lock? Are there other places (intrinsic monitors, say)
where we should do this?

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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