From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752987AbXDIGJt (ORCPT ); Mon, 9 Apr 2007 02:09:49 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752990AbXDIGJt (ORCPT ); Mon, 9 Apr 2007 02:09:49 -0400 Received: from smtp.osdl.org ([65.172.181.24]:36836 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752988AbXDIGJs (ORCPT ); Mon, 9 Apr 2007 02:09:48 -0400 Date: Sun, 8 Apr 2007 23:09:27 -0700 From: Andrew Morton To: Satoru Takeuchi Cc: Ingo Molnar , Linux Kernel , Con Kolivas , Mike Galbraith Subject: Re: [BUG] scheduler: first timeslice of the exiting thread Message-Id: <20070408230927.9a49c504.akpm@linux-foundation.org> In-Reply-To: <877isozllw.wl%takeuchi_satoru@jp.fujitsu.com> References: <877isozllw.wl%takeuchi_satoru@jp.fujitsu.com> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 07 Apr 2007 16:31:39 +0900 Satoru Takeuchi wrote: > When I was examining the following program ... > > 1. There are a large amount of small jobs takes several msecs, > and the number of job increases constantly. > 2. The process creates a thread or a process per job (I examined both > the thread model and the process model). > 3. Each child process/thread does the assigned job and exit immediately. > > ... I found that the thread model's latency is longer than proess > model's one against my expectation. It's because of the current > sched_fork()/sched_exit() implementation as follows: > > a) On sched_fork, the creator share its timeslice with new process. > b) On sched_exit, if the exiting process didn't exhaust its first > timeslice yet, it gives its timeslice to the parent. > > It has no problem on the process model since the creator is the parent. > However, on the thread model, the creator is not the parent, it is same > as the creator's parent. Hence, on this kind of program, the creator > can't retrieve shared timeslice and exausts its timeslice at a rate of > knots. In addition, somehow, the parent (typically shell?) gets extra > timeslice. > > I believe it's a bug and the exiting process should give its timeslice > to the creator. Now I have some patch plan to fix this problem as follow: > > a) Add the field for the creator to task_struct. It needs extra memory. > b) Doesn't add extra field and have thread's parent the creater, which is > same as process creation. However it has many side effects, for example, > we also need to change sys_getppid() implementation. > > What do you think? Any comments are welcome. This comes at an awkward time, because we might well merge the staircase/deadline work into 2.6.22, and I think it rewrites the part of the scheduler which is causing the problems you're observing. Has anyone verified that SD fixes this problem and the one at http://lkml.org/lkml/2007/4/7/21 ?