Re: kernel/scheduler: The Linux scheduler doesn't scale to more than 8 cores?

[Peter Zijlstra]

On Tue, Nov 02, 2021 at 04:25:05PM +0100, Vincent Guittot wrote:
> > From the first line, the CPU count is capped to 8.
> >
> > Thus the scheduler scaling won't be more than a factor of 4, no matter how many CPUs there are. (1+log2(8))
> >
> > There's also a linear scaling option, that is similarly capped to 8 times, because 8 CPUs.
> 
> That's a good point and I never realized this limitation in the scaling factor.
> 
> Peter mentioned that there were interactivity problems with large
> scale factors at the time it was added. But the scheduler has changed
> since and it would be interesting to run benchmarks on more recent
> platforms with larger factor

The important note here is that this limit is about timescales, not
anything else. Previously a 64 cpu system would blow up the latency to
6ms * (1 + ln)2(64)) = 42ms, which is atrocious.