RFC: AArch64: Implementing spin pauses with ISB

[Hohensee, Paul]

What if we made using ISB conditional on the implementation, in this case N1?

Thanks,
Paul

-----Original Message-----
From: hotspot-dev <hotspot-dev-retn at openjdk.java.net> on behalf of Andrew Haley <aph-open at littlepinkcloud.com>
Date: Tuesday, August 17, 2021 at 1:26 AM
To: "hotspot-dev at openjdk.java.net" <hotspot-dev at openjdk.java.net>
Subject: Re: RFC: AArch64: Implementing spin pauses with ISB

On 8/16/21 11:39 PM, Stuart Monteith wrote:

>       This is interesting, and thank you for bringing it up for discussion here. The ISB instruction wasn't intended to be
> used for that purpose, so while you can measure a benefit for now, there is no guarantee that it would continue to be
> beneficial in the future. I hate to suggest adding more flags, but we ought to consider adding one to disable the ISB
> instruction in the spins. The counter argument is of course that we'd update the implementation as new cores come out.

Indeed. It all sounds a bit like witchcraft to me. I guess that any instruction
which caused a significant stall would reduce contention, but it'd take some
experiments to confirm that.

Evgeny, did anyone try waiting on the monitor location with a WFE? That'd at
least be using an instruction as intended, and would certainly reduce contention,
but WFE might be too drastic for a brief spin lock.

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