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Date: Tue, 5 Oct 2021 16:32:28 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Phil Auld <pauld@...hat.com> Cc: Vincent Guittot <vincent.guittot@...aro.org>, Mike Galbraith <efault@....de>, Mel Gorman <mgorman@...hsingularity.net>, Ingo Molnar <mingo@...nel.org>, Valentin Schneider <valentin.schneider@....com>, Aubrey Li <aubrey.li@...ux.intel.com>, Barry Song <song.bao.hua@...ilicon.com>, Srikar Dronamraju <srikar@...ux.vnet.ibm.com>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to nr_running On Tue, Oct 05, 2021 at 10:12:12AM -0400, Phil Auld wrote: > On Tue, Oct 05, 2021 at 12:36:22PM +0200 Peter Zijlstra wrote: > > On Thu, Sep 23, 2021 at 08:24:03AM -0400, Phil Auld wrote: > > > > > It's capped at 8 cpus, which is pretty easy to reach these days, so the > > > values don't get too large. That scaling is almost a no-op these days. > > > > https://lkml.kernel.org/r/YVwdrh5pg0zSv2/b@hirez.programming.kicks-ass.net > > > > Ooh, hey, we already fixed that :-) > > > > Thanks Peter. > > I'm always a little behind upstream (nature of the job :) > > That link leads to a message Id not found. https://lore.kernel.org/all/YVwblBZ9JBn9vvVr@hirez.programming.kicks-ass.net/T/#u Seems to work, I must've messed up the copy/paste or something. > But from what I can see the code that takes the min of online cpus and > 8 is still present. Yes, and it should be. I was the confused one. I forgot we added it and suggested we should add it again :-) > > So the reasoning there is that if the values get too big, interactiviy > > get *really* bad, but if you go from say 1 to 4 CPUs, interactivity can > > improve due to being able to run on other CPUs. > > > > At 8 CPUs we end up at 6ms*4=24ms, which is already pretty terrible. > > > > And actually you mention the same thing later on. Most systems, even > desktops, have 8+ cpus these days so the scaling is mostly not doing > anything except multiplying by 4, right? So no-op was not the right > way to describe it maybe. But it's not getting bigger with larger > numbers of cpus beyond a pretty commonly reached limit. Yeah, the whole scaling thing is of dubious value these days, the whole 1-8 range is for embedded stuff these days, I mean, only low-end phones are maybe even still in that range -- oh and my laptop.. :/
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