From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f47.google.com (mail-ee0-f47.google.com [74.125.83.47]) by kanga.kvack.org (Postfix) with ESMTP id 56AC06B0039 for ; Fri, 9 May 2014 11:05:15 -0400 (EDT) Received: by mail-ee0-f47.google.com with SMTP id c13so2771769eek.6 for ; Fri, 09 May 2014 08:05:14 -0700 (PDT) Received: from Galois.linutronix.de (Galois.linutronix.de. [2001:470:1f0b:db:abcd:42:0:1]) by mx.google.com with ESMTPS id o49si4334155eef.248.2014.05.09.08.05.13 for (version=TLSv1.2 cipher=RC4-SHA bits=128/128); Fri, 09 May 2014 08:05:14 -0700 (PDT) Date: Fri, 9 May 2014 17:05:12 +0200 (CEST) From: Thomas Gleixner Subject: Re: vmstat: On demand vmstat workers V4 In-Reply-To: Message-ID: References: <20140508142903.c2ef166c95d2b8acd0d7ea7d@linux-foundation.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: Andrew Morton , Gilad Ben-Yossef , Tejun Heo , John Stultz , Mike Frysinger , Minchan Kim , Hakan Akkan , Max Krasnyansky , Frederic Weisbecker , "Paul E. McKenney" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, hughd@google.com, viresh.kumar@linaro.org On Fri, 9 May 2014, Christoph Lameter wrote: > On Fri, 9 May 2014, Thomas Gleixner wrote: > > I think we agreed long ago, that for the whole HPC FULL_NOHZ stuff you > > have to sacrify at least one CPU for housekeeping purposes of all > > kinds, timekeeping, statistics and whatever. > > Ok how do I figure out that cpu? I'd rather have a specific cpu that > never changes. I followed the full nohz development only losely, but back then when all started here at my place with frederic, we had a way to define the housekeeper cpu. I think we lazily had it hardwired to 0 :) That probably changed, but I'm sure there is still a way to define a housekeeper. And we should simply force the timekeeping on that housekeeper. That comes with the price, that the housekeeper is not allowed to go deep idle, but I bet that in HPC scenarios this does not matter at all simply because the whole machine is under full load. Frederic? > > So if you have a housekeeper, then it makes absolutely no sense at all > > to move it around in circles. > > > > Can you please enlighten me why we need this at all? > > The vmstat kworker thread checks every 2 seconds if there are vmstat > updates that need to be folded into the global statistics. This is not > necessary if the application is running and no OS services are being used. > Thus we could switch off vmstat updates and avoid taking the processor > away from the application. > > This has also been noted by multiple other people at was brought up at the > mm summit by others who noted the same issues. I understand why you want to get this done by a housekeeper, I just did not understand why we need this whole move it around business is required. Thanks, tglx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org