From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37491) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z79vG-0006Ru-1t for qemu-devel@nongnu.org; Mon, 22 Jun 2015 18:10:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z79vC-0002V7-OY for qemu-devel@nongnu.org; Mon, 22 Jun 2015 18:10:33 -0400 Received: from greensocs.com ([193.104.36.180]:46380) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z79vC-0002S7-H2 for qemu-devel@nongnu.org; Mon, 22 Jun 2015 18:10:30 -0400 Message-ID: <558887D2.8070009@greensocs.com> Date: Tue, 23 Jun 2015 00:10:26 +0200 From: Frederic Konrad MIME-Version: 1.0 References: <5588517C.9000909@greensocs.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] 4 CPUs with vexpress is slow? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: =?UTF-8?B?QWxleCBCZW5uw6ll?= , qemu-devel On 22/06/2015 22:57, Peter Maydell wrote: > On 22 June 2015 at 19:18, Frederic Konrad wrote: >> Testing MTTCG patch-set performance I found strange slowness with _upstream_ >> qemu (46bca5404b08201bb9df1ac32bc88fc7e6db1f74). >> >> Basically booting a vexpress-a15 with "-smp 4" takes approximately forever >> if I use a >> vexpress-v2p-ca15-tc1.dtb (2x A15) dtb and 39 secs if I hack this dtb to >> have 4 CPUs. > Wait, so if you change the dtb it boots in a non-infinite time, > but MTTCG boots both dtbs? Yes MTTCG boots both dtbs. >> This is definitely strange because if I use "-smp 2" with the same guest >> image it takes >> only 4 secs. And MTTCG patch-set seems to fix the issue as it boot in 6 secs >> with >> "-smp 4". >> >> Is that a known issue or maybe it's my guest (linux 3.13.5)? > Dunno. I basically never run with SMP configs because they're > always slower than uniprocessor... Does your kernel boot on > KVM? It may be that it's configured to assume 2xSMP somehow, > given the h/w we're modelling here is 2xSMP. > > Important question: when you boot this config, does /proc/cpuinfo > say you have two cores booted, or all four? If we've ended up with > the other two cores in the config spinning then that would explain > why MTTCG is doing better here, but it's not a very interesting > config to try to optimise :-) All four cores are in /proc/cpuinfo. > > There is an issue where IPIs may be inefficient: > http://lists.nongnu.org/archive/html/qemu-devel/2015-06/msg03824.html > but I'd expect that to merely slow things down a bit, not send > the boot time off to infinity. This is definitely a bug : Seems that removing "--nographic" option removes the problem it just boots as per normal.. Fred > > -- PMM