All of lore.kernel.org
 help / color / mirror / Atom feed
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: "Morotti, Romain (London)" <romain.morotti@man.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: kernel/scheduler: The Linux scheduler doesn't scale to more than 8 cores?
Date: Tue, 2 Nov 2021 17:04:02 +0100	[thread overview]
Message-ID: <20211102160402.GX174703@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <CAKfTPtAKpMj15dHO1MC=dH_XJQe1Os24k93N2jDZ=kgg3O7K7A@mail.gmail.com>

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.


      reply	other threads:[~2021-11-02 16:54 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <6725b3093d47459fa438825fb187970f@man.com>
2021-11-02 15:25 ` kernel/scheduler: The Linux scheduler doesn't scale to more than 8 cores? Vincent Guittot
2021-11-02 16:04   ` Peter Zijlstra [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20211102160402.GX174703@worktop.programming.kicks-ass.net \
    --to=peterz@infradead.org \
    --cc=juri.lelli@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=romain.morotti@man.com \
    --cc=vincent.guittot@linaro.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.