From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758605AbYETEwn (ORCPT ); Tue, 20 May 2008 00:52:43 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751694AbYETEwd (ORCPT ); Tue, 20 May 2008 00:52:33 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.125]:51606 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751423AbYETEwc (ORCPT ); Tue, 20 May 2008 00:52:32 -0400 Date: Tue, 20 May 2008 00:37:02 -0400 (EDT) From: Steven Rostedt X-X-Sender: rostedt@gandalf.stny.rr.com To: MA QING A cc: Kasper Sandberg , LKML , RT , Ingo Molnar , Thomas Gleixner Subject: RE: 2.6.25.4-rt2 In-Reply-To: <630E9048ECEBBB4798CE33255209FE730648317D@CNSHGSMBS02.ad4.ad.alcatel.com> Message-ID: References: <1211237032.14697.76.camel@localhost.localdomain><1211239147.198 45.42.camel@localhost> <630E9048ECEBBB4798CE33255209FE730648317D@CNSHGSMBS02.ad4.ad.alcatel.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 20 May 2008, MA QING A wrote: > Forgive me to ask a stupid question. > Why using the 24 mainline kernel, the time is less than using the > rt-patched kernel? > The test I used is a performance test, not a latency test. An RTOS (Real-time Operating System) will sacrifice performance to achieve determinism (low latencies). Several key features to an RT system usually come with a performance cost. A non RT system will perform 99% of the time faster than an RTOS. But all it takes is that one time to miss a deadline to make an RT system crash. An RTOS may be slightly slower, but it will not have those outliers that a normal desktop system would have. So getting back to your question. Hackbench runs a bunch of stuff and times how long it took to do so. It stresses the system quite a bit. The lower the number the better. Given that hackbench is not checking for latency, but only shear perforance, it is expected that hackbench will run better on a non RT system. Now, a test like cyclictest that measures latencies will show the benefits of realtime. The kernel with the RT patch can measure 65 microsecond latencies for response times even while running hackbench. The vanilla kernel would measure a few milliseconds response times running the same test. -- Steve