Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753753AbcKYQKK (ORCPT ); Fri, 25 Nov 2016 11:10:10 -0500 Received: from mail-wm0-f65.google.com ([74.125.82.65]:35624 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754743AbcKYQJj (ORCPT ); Fri, 25 Nov 2016 11:09:39 -0500 Subject: Re: RFC: documentation of the autogroup feature [v2] To: Mike Galbraith References: <41d802dc-873a-ff02-17ff-93ce50f3e925@gmail.com> <1479901185.4306.38.camel@gmx.de> <327586fa-4672-d070-0ded-850654586273@gmail.com> <1479915229.4306.106.camel@gmx.de> <7513b0a5-c5d0-3a92-5849-995af22601e4@gmail.com> <1479921075.4306.153.camel@gmx.de> <1480078973.4075.58.camel@gmx.de> <1480089063.4075.80.camel@gmx.de> Cc: mtk.manpages@gmail.com, Peter Zijlstra , Ingo Molnar , linux-man , lkml , Thomas Gleixner From: "Michael Kerrisk (man-pages)" Message-ID: Date: Fri, 25 Nov 2016 17:08:44 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <1480089063.4075.80.camel@gmx.de> Content-Type: text/plain; charset=iso-2022-jp Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3039 Lines: 60 On 11/25/2016 04:51 PM, Mike Galbraith wrote: > On Fri, 2016-11-25 at 16:04 +0100, Michael Kerrisk (man-pages) wrote: > >>>> ┌─────────────────────────────────────────────────────┐ >>>> │FIXME │ >>>> ├─────────────────────────────────────────────────────┤ >>>> │How do the nice value of a process and the nice │ >>>> │value of an autogroup interact? Which has priority? │ >>>> │ │ >>>> │It *appears* that the autogroup nice value is used │ >>>> │for CPU distribution between task groups, and that │ >>>> │the process nice value has no effect there. (I.e., │ >>>> │suppose two autogroups each contain a CPU-bound │ >>>> │process, with one process having nice==0 and the │ >>>> │other having nice==19. It appears that they each │ >>>> │get 50% of the CPU.) It appears that the process │ >>>> │nice value has effect only with respect to schedul‐ │ >>>> │ing relative to other processes in the *same* auto‐ │ >>>> │group. Is this correct? │ >>>> └─────────────────────────────────────────────────────┘ >>> >>> Yup, entity nice level affects distribution among peer entities. >> >> Huh! I only just learned about this via my experiments while >> investigating autogroups. >> >> How long have things been like this? Always? (I don't think >> so.) Since the arrival of CFS? Since the arrival of >> autogrouping? (I'm guessing not.) Since some other point? >> (When?) > > Always. Before CFS there just were no non-peers :) Well that's one way of looking at it. So, the change that I'm talking about came in 2.6.32 with CFS then? >> It seems to me that this renders the traditional process >> nice pretty much useless. (I bet I'm not the only one who'd >> be surprised by the current behavior.) > > Yup, group scheduling is not a single edged sword, those don't exist. > Box wide nice loss is not the only thing that can bite you, fairness, > whether group or task oriented cuts both ways. Understood. But again I'll say, I bet a lot of old-time users (and maybe many newer) would be surprised by the fact that nice(1) / setpriority(2) have effectively been rendered no-ops in many use cases. At the very least, it'd have been nice if someone had sent a man pages patch or at least a note... Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/