From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: I/O performance of VirtIO Date: Sun, 25 Oct 2009 07:44:16 +0200 Message-ID: <4AE3E5B0.5080700@redhat.com> References: <20091012204901.GA10688@nightfall.luchs.at> <4AD3A38D.3090102@msgid.tls.msk.ru> <4AD41FB8.6000209@web.de> <4AE0885A.1080304@redhat.com> <7A331778-3227-43B1-B2E2-A6BE6F92C834@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Jan Kiszka , Michael Tokarev , =?UTF-8?B?UmVuw6kgUGZlaWZmZXI=?= , "kvm@vger.kernel.org" To: Alexander Graf Return-path: Received: from mx1.redhat.com ([209.132.183.28]:19252 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752712AbZJYFpN (ORCPT ); Sun, 25 Oct 2009 01:45:13 -0400 In-Reply-To: <7A331778-3227-43B1-B2E2-A6BE6F92C834@suse.de> Sender: kvm-owner@vger.kernel.org List-ID: On 10/23/2009 12:06 AM, Alexander Graf wrote: > > Am 22.10.2009 um 18:29 schrieb Avi Kivity : > >> On 10/13/2009 08:35 AM, Jan Kiszka wrote: >>> It can be particularly slow if you use in-kernel irqchips and the >>> default NIC emulation (up to 10 times slower), some effect I always >>> wanted to understand on a rainy day. So, when you actually want -net >>> user, try -no-kvm-irqchip. >>> >> >> This might be due to a missing SIGIO or SIGALRM; -no-kvm-irqchip >> generates a lot of extra signals and thus polling opportunities. > > Isn't that what dedicated io threads are supposed to solve? > No. Dedicated I/O threads provide parallelism. All latency needs is to have SIGIO sent on all file descriptors (or rather, in qemu-kvm with irqchip, to have all file descriptors in the poll() call). Jan, does slirp add new connections to the select set? -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.