From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: major slowdown in i/o (due to high CPU usage) from 2.6.32 to current Date: Tue, 19 Jul 2011 12:14:36 +0300 Message-ID: <4E254AFC.8040405@redhat.com> References: <4E249499.4050502@msgid.tls.msk.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: KVM list , Svante Signell , Gleb Natapov To: Michael Tokarev Return-path: Received: from mx1.redhat.com ([209.132.183.28]:21881 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752419Ab1GSJPA (ORCPT ); Tue, 19 Jul 2011 05:15:00 -0400 In-Reply-To: <4E249499.4050502@msgid.tls.msk.ru> Sender: kvm-owner@vger.kernel.org List-ID: On 07/18/2011 11:16 PM, Michael Tokarev wrote: > It all started by Svante Signell on IRC, see > http://bugs.debian.org/634149 . The short story: > running hurd guest image (on ide drive); when changing > only host kernel from 2.6.32 to (in my case) 3.0.0-rc7, > the i/o (especially write) speed of the guest reduces > dramatically, as is host CPU usage. I used a cd-1.iso > install image from the URL provided in the bugreport, > http://people.debian.org/~sthibault/hurd-i386/installer/cdimage/ -- > even installing packages is very significantly slower > in current kernels than it was in 2.6.32. > > Typical kvm_stats output on 3.0.0 host kernel: > > kvm-18166 [001] 47471.398406: kvm_emulate_insn: 80000000:160750: f3 66 6f > kvm-18166 [001] 47471.398406: kvm_pio: pio_write at 0x1f0 size 2 count 1 > kvm-18166 [001] 47471.398407: kvm_userspace_exit: reason KVM_EXIT_IO (2) > kvm-18166 [001] 47471.398408: kvm_entry: vcpu 0 > kvm-18166 [001] 47471.398409: kvm_exit: reason EXIT_IOIO rip 0x160750 info 1f0012c 0 > > > Gleb, Avi -- what we're missing here? > > It does not look like the problem we have with FreeBSD, > this is apparently something different... Looks like hurd doesn't use DMA. Instead it uses rep/outsw, which indeed has a performance regression - it now exits on every word instead of on every sector. Gleb? -- error compiling committee.c: too many arguments to function