From mboxrd@z Thu Jan 1 00:00:00 1970 From: Grant Edwards Subject: Re: Using patch-2.6.33.7.2-rt30 increases latency and CPU usage? Date: Wed, 9 May 2012 22:00:00 +0000 (UTC) Message-ID: References: To: linux-rt-users@vger.kernel.org Return-path: Received: from plane.gmane.org ([80.91.229.3]:47887 "EHLO plane.gmane.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759747Ab2EIWAP (ORCPT ); Wed, 9 May 2012 18:00:15 -0400 Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1SSEvd-0007Xm-JB for linux-rt-users@vger.kernel.org; Thu, 10 May 2012 00:00:13 +0200 Received: from dsl.comtrol.com ([64.122.56.22]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 10 May 2012 00:00:13 +0200 Received: from grant.b.edwards by dsl.comtrol.com with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 10 May 2012 00:00:13 +0200 Sender: linux-rt-users-owner@vger.kernel.org List-ID: On 2012-05-09, Grant Edwards wrote: > IOW: the real-time patch for 2.6.33.7 makes both the typical interrupt > latency and the CPU usage significantly worse. > > Typical latency without the RT patch is 5-15us. > > Typical latency with is 15-50us (I've never seen latency below 15us > with the RT patch). > > Is this the expected behavior? I've been loaned a clue by somebody on the OSADL mailing list: the RT patches are for improving _user_space_ reponse, and may do so at the expense of both CPU usage and interrupt latency. As a result, I'm better off without the RT patch if what I care about is interrupt latency. Am right I correct in the following conclusions? * In 2.6.33.7, ISRs are really ISRs * In 2.6.33.7-rt30, ISRs are kernel threads. -- Grant Edwards grant.b.edwards Yow! Don't hit me!! I'm in at the Twilight Zone!!! gmail.com