From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=42163 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PmlY8-0008Pl-Oz for qemu-devel@nongnu.org; Tue, 08 Feb 2011 06:16:01 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PmlY7-0006U7-M4 for qemu-devel@nongnu.org; Tue, 08 Feb 2011 06:16:00 -0500 Received: from hall.aurel32.net ([88.191.126.93]:45238) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PmlY7-0006Tz-Eo for qemu-devel@nongnu.org; Tue, 08 Feb 2011 06:15:59 -0500 Message-ID: <4D5125E2.8090902@aurel32.net> Date: Tue, 08 Feb 2011 12:15:46 +0100 From: Aurelien Jarno MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH 2/7] Enable I/O thread and VNC threads by default References: <4D3DFD20.8060004@linux.vnet.ibm.com> <20110125091741.GB30239@edde.se.axis.com> <20110125133453.GC5427@amt.cnet> <20110207101255.GA20413@amt.cnet> <20110207160350.GA26332@amt.cnet> <4D501C71.7090708@redhat.com> <4D50279B.5010102@siemens.com> <4D505DCB.9050406@codemonkey.ws> <20110207214551.GB16429@hall.aurel32.net> <4D50A5F0.802@codemonkey.ws> <20110208072657.GD16429@hall.aurel32.net> <4D50FA14.5010100@redhat.com> <4D5103E8.6050808@siemens.com> <4D510771.3040309@aurel32.net> <4D511221.9030505@siemens.com> <4D5113D3.9090802@aurel32.net> <4D511500.1040303@siemens.com> <4D5115C2.6060008@aurel32.net> <4D51842C.8000209@codemonkey.ws> In-Reply-To: <4D51842C.8000209@codemonkey.ws> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 8bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: Stefan Hajnoczi , Jan Kiszka , Marcelo Tosatti , "qemu-devel@nongnu.org" , Anthony Liguori , Paul Brook , Paolo Bonzini , "Edgar E. Iglesias" , Arun Bharadwaj Anthony Liguori a écrit : > On 02/08/2011 04:06 AM, Aurelien Jarno wrote: >> Yes, it's slow. But is it a problem? You assume that people use QEMU >> only for emulating SMP platforms. This is a wrong assumption. Beside the >> x86 target, only sparc really supports SMP emulation. >> > > It's *not* just about performance. > > TCG requires a signal to break out of a tight chained TB loop. If you > have a guest in a tight loop waiting for something external (like > polling on a in-memory flag), the device emulation will not get to run > until a signal is fired. > > Unless you set SIGIO on every file descriptor that selects polls on (and > you can't because there are a number that just don't support SIGIO), > then you have a race condition. > In practice you will get a signal when the next timer event expire. I agree it's suboptimal, but it works, and has been like that for here. Having that fixed through an I/O thread is actually quite nice, however it should not be done ignoring all the *current* drawbacks of the iothread mode. We know them (at least for some of them), so let's try to solve them. And now, I don't buy the argument "it's been there for years", it was *disabled* by default. -- Aurelien Jarno GPG: 1024D/F1BCDB73 aurelien@aurel32.net http://www.aurel32.net