From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [RFT] mmu optimizations branch Date: Wed, 3 Jan 2007 03:22:41 +0100 Message-ID: <20070103022241.GA13840@elte.hu> References: <4598E33B.608@qumranet.com> <20070102161117.GA3306@elte.hu> <459A8909.7020600@qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel Return-path: To: Avi Kivity Content-Disposition: inline In-Reply-To: <459A8909.7020600-atKUWr5tajBWk0Htik3J/w@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 * Avi Kivity wrote: > >lat_ctx -s 0 [zero memory footprint]: > > > > ------------------------------------------------- > > #tasks native kvm-r4204 kvm-r4232(mmu) > > ------------------------------------------------- > > 2: 2.02 180.91 9.19 > > 20: 4.04 183.21 10.01 > > 50: 4.30 185.95 11.27 > > > >so here it's a /massive/, almost 20 times speedup! > > > > Excellent. 10us is approximately the vmexit overhead on intel (we > regularly see 100-120k exits/sec), so it means a context switch is > exactly one exit. Hard to beat without nested page tables. actually, the VM entry+exit cost on this CPU is around 3-4 microseconds, so it's still 2 VM exits per context switch. I debugged this a bit, and what happens is that when Linux does a task-switch it does a cr3 load /and/ a write (look at __flush_tlb()) - and both are causing a vm exit! I have started paravirtualizing the Linux kernel for KVM. I have eliminated the cr3 load from the Linux kernel via paravirtualization and that way lat_ctx shows a ~5-6 usecs context-switch cost. That's pretty good i think, compared to the 2-3 usecs of native. I'll send patches for this tomorrow. Ingo ------------------------------------------------------------------------- Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys - and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV