From mboxrd@z Thu Jan 1 00:00:00 1970 From: Agustin Ferrin Pozuelo Subject: Re: 2.6.33.7-rt29 PREEMPT_RT worse latency than PREEMPT_DESKTOP on AT91? Date: Wed, 15 Sep 2010 16:42:17 +0100 Message-ID: <4C90E959.80306@cgglobal.com> References: <4C72A438.2080301@cgglobal.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: "linux-rt-users@vger.kernel.org" To: Thomas Gleixner Return-path: Received: from edge.cgglobal.com ([202.54.16.167]:55006 "EHLO edge.cgglobal.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753682Ab0IOPmZ (ORCPT ); Wed, 15 Sep 2010 11:42:25 -0400 Received: from cgglobal.com (webmail.cgl.co.in [192.168.1.7] (may be forged)) by edge.cgglobal.com (8.14.1/8.14.1) with ESMTP id o8FFc7ul032478 for ; Wed, 15 Sep 2010 21:08:07 +0530 Received: from mail.microsol.ie by cgglobal.com (MDaemon PRO v9.6.6) with ESMTP id md50027273624.msg for ; Wed, 15 Sep 2010 21:11:48 +0530 In-Reply-To: Sender: linux-rt-users-owner@vger.kernel.org List-ID: On 27/08/10 11:35, Thomas Gleixner wrote: > On Mon, 23 Aug 2010, Agustin Ferrin Pozuelo wrote: >> Hello, >> >> I am fine tuning the configuration for an ARM system derived from >> AT91SAM9263-EK. >> >> My goal is to minimize latency, and I am using "cyclictest" from rt-= tools >> v0.78 for measuring it. >> >> I get consistently better latency with PREEMPT_DESKTOP over what I g= et with >> PREEMPT_RT. This is an example for a very simple test run, which ref= lects the >> overall results I am getting: >> >> ### PREEMPT-RT, HRT, no NO_HZ, no RTC, no USE_SLOW_CLOCK >> >> root@at91sam9263cpc:~# time cyclictest -i 700000 -r -p 80 -l 33 >> Clock resolution: 0.000000001 (s.ns) >> policy: fifo: loadavg: 0.11 0.10 0.04 1/49 884 >> >> T: 0 ( 861) P:80 I:700000 C: 33 Min: 370 Act: 627 Avg: = 437 Max: >> 627 >> real 0m 23.51s >> user 0m 0.38s >> sys 0m 2.20s >> >> ### PREEMPT-DESKTOP (no RT), HRT, no NO_HZ, no USE_SLOW_CLOCK >> root@at91sam9263cpc:~# time cyclictest -i 700000 -r -p 80 -l 33 >> Clock resolution: 0.000000001 (s.ns) >> policy: fifo: loadavg: 0.24 0.08 0.03 1/46 600 >> >> T: 0 ( 574) P:80 I:700000 C: 33 Min: 173 Act: 196 Avg: = 222 Max: >> 378 >> real 0m 23.66s >> user 0m 0.32s >> sys 0m 1.29s > 33 loops are not really giving you any useful information. Also run > both tests with some background load and not on a fully idle system. Sorry, my test setup is not very comprehensive at the moment. I have=20 some overnight/overweekend results at hand with millions of loops: PREEMPT-DESKTOP: root@at91sam9263cpc:~# time cyclictest -i 70000 -p 80 -h 700 -r Clock resolution: 0.000000001 (s.ns) policy: fifo: loadavg: 0.07 0.09 0.02 1/38 8770 T: 0 ( 980) P:80 I:70000 C:1328241 Min: 135 Act: 218 Avg: 20= 4=20 Max: 720 PREEMPT-RT: root@at91sam9263cpc:~# time cyclictest -i 70000 -p 80 -h 700 -r Clock resolution: 0.000000001 (s.ns) policy: fifo: loadavg: 0.97 1.12 1.18 4/51 31290 T: 0 ( 675) P:80 I:70000 C:3688312 Min: 244 Act: 358 Avg: =20 395 Max: 732 Min and avg latencies much worse in PREEMPT-RT on otherwise same setup.= =20 Worst case similar though. System is not loaded but many interrupts happen because it is running i= t=20 interactively through ssh+ethernet. (Had to force ethernet to 10Mbit to= =20 avoid frequent buffer underruns due to sam9263 bug). I am assuming context switching is more expensive on PREEMPT-RT under=20 ARM9, where it seems already a bit expensive. --Agust=EDn. --=20 [CG logo] Agust=EDn Ferr=EDn Pozuelo Embedded Systems Engineer CG Power Systems Ireland Limited Automation Systems Division Herbert House, Harmony Row, Dublin 2, Ireland. Phone: +353 1 4153700 Web: www.cgglobal.com Save the environment. Please print only if essential. CG DISCLAIMER: This email contains confidential information. It is inte= nded exclusively for the addressees. If you are not an addressee, you m= ust not store, transmit or disclose its contents. Instead please notify= the sender immediately; and permanently delete this e-mail from your c= omputer systems. We have taken reasonable precautions to ensure that no= viruses are present. However, you must check this email and the attach= ments, for viruses. We accept no liability whatsoever, for any detrimen= t caused by any transmitted virus. -- To unsubscribe from this list: send the line "unsubscribe linux-rt-user= s" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html