From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BCD6EC43219 for ; Tue, 2 Nov 2021 16:54:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9DEE560F45 for ; Tue, 2 Nov 2021 16:54:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234967AbhKBQ5N (ORCPT ); Tue, 2 Nov 2021 12:57:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46774 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230008AbhKBQ46 (ORCPT ); Tue, 2 Nov 2021 12:56:58 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4FDA6C06120B for ; Tue, 2 Nov 2021 09:04:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=1sJdvJqhAZ8971iSxAtIuMLFnEJHx4QlcRhowz6J5Po=; b=UdvDzwHUdl7TAS3z1mhORKh/yt 3irrVNIEUFmZ5bDiNfBaX5LloCH9pcJQMb13F3WP2htvyofIRD+GTlIACkkPU8vClmBBZ5N9LxK9d s/oAa77XtKdncaRdHRAPa3XlOrndiCrW7tSnA6HWW37m3/SHjTh2VyRnpDSDo3eLdHBUMLOWTVs6C jhuBuVgwEXyzPezZiXWsmxfWtfEYicLPe4gj4JaDcfDLax+hLjuA9aCyse4PSa9GweF2DDdHnVZD9 MrtJmT0TbDo7NW4qa+rFDhbn6Kr2WqaTKNn2l+Htr6snw6iWFBgxwMoSudUG3oNvfMTCRFB7jUk6/ z36oG+ZQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mhwGa-00DmKw-1A; Tue, 02 Nov 2021 16:04:04 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id C25B39816BA; Tue, 2 Nov 2021 17:04:02 +0100 (CET) Date: Tue, 2 Nov 2021 17:04:02 +0100 From: Peter Zijlstra To: Vincent Guittot Cc: "Morotti, Romain (London)" , Ingo Molnar , Juri Lelli , "linux-kernel@vger.kernel.org" Subject: Re: kernel/scheduler: The Linux scheduler doesn't scale to more than 8 cores? Message-ID: <20211102160402.GX174703@worktop.programming.kicks-ass.net> References: <6725b3093d47459fa438825fb187970f@man.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.