From mboxrd@z Thu Jan 1 00:00:00 1970 From: Agustin Ferrin Pozuelo Subject: 2.6.33.7-rt29 PREEMPT_RT worse latency than PREEMPT_DESKTOP on AT91? Date: Mon, 23 Aug 2010 17:39:20 +0100 Message-ID: <4C72A438.2080301@cgglobal.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE To: Return-path: Received: from edge.cgglobal.com ([202.54.16.167]:51626 "EHLO edge.cgglobal.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751579Ab0HWQjf (ORCPT ); Mon, 23 Aug 2010 12:39:35 -0400 Received: from cgglobal.com ([192.168.1.7]) by edge.cgglobal.com (8.14.1/8.14.1) with ESMTP id o7NGdVc6005311 for ; Mon, 23 Aug 2010 22:09:31 +0530 Received: from mail.microsol.ie by cgglobal.com (MDaemon PRO v9.6.6) with ESMTP id md50026552261.msg for ; Mon, 23 Aug 2010 22:08:44 +0530 Sender: linux-rt-users-owner@vger.kernel.org List-ID: Hello, I am fine tuning the configuration for an ARM system derived from=20 AT91SAM9263-EK. My goal is to minimize latency, and I am using "cyclictest" from=20 rt-tools v0.78 for measuring it. I get consistently better latency with PREEMPT_DESKTOP over what I get=20 with PREEMPT_RT. This is an example for a very simple test run, which=20 reflects 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: 43= 7 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: 22= 2 Max: 378 real 0m 23.66s user 0m 0.32s sys 0m 1.29s =20 That is, I get much better min, avg, and max latency with=20 "PREEMPT_DESKTOP". How can this be? Anyway, the results seem worse than the <150us worst case) obtained wit= h=20 2.6.24.7-rt27 by Remy Bohmer=20 ,=20 though he was apparently/ not using 'Tickless and HRT' /because of CPU=20 usage issues... But without those I can't get cyclictest to run properl= y! My guess is that the RT patch adds code (checks, locks) to may critical= =20 sections that happens to be specially slow on ARM9... Clues & comments welcome! --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