From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [Qemu-devel] Re: [PATCH 0/4] Rework alarm timer infrastrucure - take2 Date: Sun, 19 Aug 2007 22:30:00 +0300 Message-ID: <46C89A38.60509@qumranet.com> References: <20070817231149.544849769@gmail.com> <20070819131042.GA22798@mail.shareable.org> <46C84A16.7040305@qumranet.com> <200708191457.21237.paul@codesourcery.com> <20070819171545.GB16928@mail.shareable.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, Paul Brook , qemu-devel-qX2TKyscuCcdnm+yROfE0A@public.gmane.org To: Jamie Lokier Return-path: In-Reply-To: <20070819171545.GB16928-tp2ajI7sM85Y6zH9YvfY1x2eb7JE58TQ@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Errors-To: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: kvm.vger.kernel.org Jamie Lokier wrote: > 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. > No, something else is happening here. On kvm, a signal is maybe 10us, to the SIGALRMs represent 1% of overhead. Respectable, but not more. I suspect that the increased accuracy causes something to behave better (or just differently). This won't be representative. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/