From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754334Ab1GIGaQ (ORCPT ); Sat, 9 Jul 2011 02:30:16 -0400 Received: from mail-pz0-f46.google.com ([209.85.210.46]:58581 "EHLO mail-pz0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752628Ab1GIGaL (ORCPT ); Sat, 9 Jul 2011 02:30:11 -0400 Date: Sat, 9 Jul 2011 16:29:48 +1000 From: Robert Swan To: linux-kernel@vger.kernel.org Subject: Re: [bisected] pty performance problem Message-ID: <20110709062948.GA8093@swanrl.gmail.com> Reply-To: swan.r.l@gmail.com References: <20091121222319.GA3905@swanrl.gmail.com> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="YiEDa0DAkWCtVeE4" Content-Disposition: inline In-Reply-To: <20091121222319.GA3905@swanrl.gmail.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --YiEDa0DAkWCtVeE4 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Sun, Nov 22, 2009 at 09:23:19AM +1100, Robert Swan wrote: > I posted this to the kernel-newbies list, but have graduated to the > adults forum: > > ! Two C programs are having a query-response conversation through a > ! pseudo terminal: > ! > ! A (client) -- forever { send query; read response } > ! B (server) -- forever { read query; send response } > ! > ! Neither has any I/O apart from the pty conversation, so I'd expect to > ! see CPU usage at 100%. When I ran it, the CPU was pretty well idle. > ! After a fair bit of fiddling, it turned out that both sides were > ! taking about 8ms for their read() calls. At that point it seemed > ! pretty clear that this was a delay in the kernel, not the code. > ! > [snip] > > 2.6.31-rc2-00205-gb4b21ca good > 2.6.31-rc2-00206-gd945cb9 bad > > and still bad with the latest: 2.6.32-rc8-00011-ga8a8a66 And still bad with the 2.6.37 release. I have been using a workaround happily enough, so I'm not nagging. 18 months hardly rates as nagging frequency, possibly sniping, but I'm not doing that either. I tried my test case for the first time in ages today and thought a reminder was merited in case it had slipped between the cracks. I'll attach the test code, since I didn't post that to the list last time. 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