Re: epoll_wait() performance

[Willy Tarreau]

On Thu, Nov 28, 2019 at 04:37:01PM +0000, David Laight wrote:
> My test system tends to increase its clock rate when busy.
> (The fans speed up immediately, the cpu has a passive heatsink and all the
> case fans are connected (via buffers) to the motherboard 'cpu fan' header.)
> I could probably work out how to lock the frequency, but for some tests I run:
> $ while :; do :; done
> Putting 1 cpu into a userspace infinite loop make them all run flat out
> (until thermally throttled).

It would be way more efficient to only make the CPUs spin in the idle
loop. I wrote a small module a few years ago for this, which allows me
to do the equivalent of "idle=poll" at runtime. It's very convenient
in VMs as it significantly reduces your latency and jitter by preventing
them from sleeping. It's quite efficient as well to stabilize CPUs having
an important difference between their highest and lowest frequencies.

I'm attaching the patch here, it's straightforward, it was made on
3.14 and still worked unmodified on 4.19, I'm sure it still does with
more recent kernels.

Hoping this helps,
Willy

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