From mboxrd@z Thu Jan 1 00:00:00 1970 From: Clark Williams Subject: Re: 1us latency? Date: Thu, 6 Aug 2015 10:03:28 -0500 Message-ID: <20150806100328.5bd71f63@sluggy> References: <55BF4CD7.8000007@pavlinux.ru> <20150803114422.0efbea1b@sluggy> <55BFAC1C.9020508@pavlinux.ru> <55BFB4AA.4020901@pavlinux.ru> <20150803135326.3fd23809@sluggy> <20150806131259.GA7924@linutronix.de> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: pavel , Linux RT Users To: Sebastian Andrzej Siewior Return-path: Received: from mx1.redhat.com ([209.132.183.28]:53412 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754621AbbHFPQn (ORCPT ); Thu, 6 Aug 2015 11:16:43 -0400 In-Reply-To: <20150806131259.GA7924@linutronix.de> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Thu, 6 Aug 2015 15:12:59 +0200 Sebastian Andrzej Siewior wrote: > * Clark Williams | 2015-08-03 13:53:26 [-0500]: > > >On Mon, 3 Aug 2015 21:36:26 +0300 > > > >Interesting. Betting that's page faults and cache filling. > > > >I don't think we want to arbitrarily pick some number of cycles for a > >"settle time" (i.e. a grace period for the application to reach steady > >state). Possibly we should add an option for that? Specify some number > >of cycles or some amount of time that where the measurement threads run > >before actual measurements start? > > > > $ cyclictest --numa -p95 -m --settle=10ms > > > >That would say "run the measurement threads for ten milliseconds before > >actually starting the measurement period". That would allow them to > >fault in and fill cache lines before starting real work. > > > >Anyone else have an opinion? > > Wouldn't you have everything in-memory after once cycle of each thread? I had to go through the timerthread() routine a couple of times to convince myself, but I think you're right. So if we wanted to discount the paging-in overhead, we could have each thread do a "dummy" pass through the timer loop (i.e. do everything but just not record the results) and then start recording measurements. I may hack together an option to try that and see what sort of results we get. Clark