From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: system call time increase when turning on CONFIG_PARAVIRT Date: Fri, 02 Mar 2007 13:54:03 -0800 Message-ID: <45E89CFB.4090905@goop.org> References: <1172866274.4898.14.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <1172866274.4898.14.camel@localhost.localdomain> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.osdl.org Errors-To: virtualization-bounces@lists.osdl.org To: tim.c.chen@linux.intel.com Cc: Virtualization Mailing List , linux-kernel@vger.kernel.org List-Id: virtualization@lists.linuxfoundation.org Tim Chen wrote: > With CONFIG_PARAVIRT turned on, I've found that time invoking > system_call jumped up quite a lot. Using TCP streaming test as a > workload and running on 32-bit 2.6.20 kernel, system_call goes up from > 0.00025% all the way to 1.6% in the oprofile data. There is a drop of > about 4% in overall throughput for this particular workload. = > > With lmbench's null system call test, the call time goes up from 0.10 > usec to 0.225 usec. > > I'm testing on dual socket Intel core 2 processor running at 2.67 GHz > with 4 GB RAM. [ I assume you're talking about running on native hardware. ] In the current paravirt changes in the kernel, many of the paravirtualized operations are implemented as (expensive) indirect calls via paravirt_ops. Among the changes in the paravirt patches I posted yesterday is an enhanced patching mechanism which inlines a lot of the common operations, and converts the rest into direct calls. I haven't done any detailed measurements on what effect this will have, but it does bring the actual executed instruction stream much closer to the !CONFIG_PARAVIRT case, and so I would hope it would recover most or all of the performance loss you've noticed. J