From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1IMoNY-0005WJ-P4 for qemu-devel@nongnu.org; Sun, 19 Aug 2007 13:15:56 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1IMoNX-0005TV-4G for qemu-devel@nongnu.org; Sun, 19 Aug 2007 13:15:56 -0400 Received: from [199.232.76.173] (helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1IMoNX-0005TL-1X for qemu-devel@nongnu.org; Sun, 19 Aug 2007 13:15:55 -0400 Received: from mail.shareable.org ([81.29.64.88]) by monty-python.gnu.org with esmtps (TLS-1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1IMoNW-0001Vz-Lx for qemu-devel@nongnu.org; Sun, 19 Aug 2007 13:15:54 -0400 Date: Sun, 19 Aug 2007 18:15:45 +0100 From: Jamie Lokier Subject: Re: [kvm-devel] [Qemu-devel] Re: [PATCH 0/4] Rework alarm timer infrastrucure - take2 Message-ID: <20070819171545.GB16928@mail.shareable.org> References: <20070817231149.544849769@gmail.com> <20070819131042.GA22798@mail.shareable.org> <46C84A16.7040305@qumranet.com> <200708191457.21237.paul@codesourcery.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200708191457.21237.paul@codesourcery.com> Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paul Brook Cc: kvm-devel@lists.sourceforge.net, qemu-devel@nongnu.org Paul Brook wrote: > > Yes, good thinking, but this should only be done if it actually impacts > > something. Reducing overhead from 0.1% to 0.05% is not worthwhile if it > > introduces extra complexity. > > If the overhead is that small, why are we touching this code in the first > place? Insightful. A benchmark result was posted which is rather interesting: >[mkent@localhost ~]$ time ./hackbench 50 >x86_64 host : real 0m10.845s >x86_64 host, bound to 1 cpu : real 0m21.884s >i386 guest+unix clock : real 0m49.206s >i386 guest+hpet clock : real 0m48.292s >i386 guest+dynticks clock : real 0m28.835s > >Results are repeatable and verfied with a stopwatch because I didn't >believe them at first :) I am surprised if 1000 redundant SIGALRMs per second is really causing 70% overhead in normal qemu execution, except on a rather old or slow machine where signal delivery is very slow. It would be good to understand the cause of that benchmark result. -- Jamie