Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763454AbXHQHXF (ORCPT ); Fri, 17 Aug 2007 03:23:05 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756849AbXHQHWz (ORCPT ); Fri, 17 Aug 2007 03:22:55 -0400 Received: from mx0.karneval.cz ([81.27.192.123]:7403 "EHLO av1.karneval.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756274AbXHQHWy (ORCPT ); Fri, 17 Aug 2007 03:22:54 -0400 X-Greylist: delayed 1347 seconds by postgrey-1.27 at vger.kernel.org; Fri, 17 Aug 2007 03:22:54 EDT Message-ID: <46C54787.1010909@stud.feec.vutbr.cz> Date: Fri, 17 Aug 2007 09:00:23 +0200 From: Michal Schmidt User-Agent: Thunderbird 2.0.0.5 (X11/20070719) MIME-Version: 1.0 To: GolovaSteek CC: linux-kernel@vger.kernel.org Subject: Re: nanosleep() accuracy References: In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1246 Lines: 38 GolovaSteek skrev: > Hello! > I need use sleep with accurat timing. > I use 2.6.21 with rt-prempt patch. > with enabled rt_preempt, dyn_ticks, and local_apic > But > > req.tv_nsec = 300000; > req.tv_sec = 0; > nanosleep(&req,NULL) > > make pause around 310-330 microseconds. How do you measure this? If you want to have something done every 300 microseconds, you must not sleep for 300 microseconds in each iteration, because you'd accumulate errors. Use a periodic timer or use the current time to compute how long to sleep in each iteration. Take a look how cyclictest does it. > I tried to understend how work nanosleep(), but it not depends from > jiffies and from smp_apic_timer_interrupt. > > When can accuracy be lost? > And how are process waked up? > > > GolovaSteek Don't forget the process will always have non-zero wakeup latency. It takes some time to process an interrupt, wakeup the process and schedule it to run on the CPU. 10-30 microseconds is not unreasonable. Michal - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/