From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=45577 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PzraQ-0008Ti-Bz for qemu-devel@nongnu.org; Wed, 16 Mar 2011 10:20:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PzraO-0003aQ-Kf for qemu-devel@nongnu.org; Wed, 16 Mar 2011 10:20:30 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:55194) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PzraO-0003aF-Cj for qemu-devel@nongnu.org; Wed, 16 Mar 2011 10:20:28 -0400 Received: from d03relay03.boulder.ibm.com (d03relay03.boulder.ibm.com [9.17.195.228]) by e37.co.us.ibm.com (8.14.4/8.13.1) with ESMTP id p2GEHmbn029892 for ; Wed, 16 Mar 2011 08:17:48 -0600 Received: from d03av02.boulder.ibm.com (d03av02.boulder.ibm.com [9.17.195.168]) by d03relay03.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p2GEKMc2089642 for ; Wed, 16 Mar 2011 08:20:23 -0600 Received: from d03av02.boulder.ibm.com (loopback [127.0.0.1]) by d03av02.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p2GEKMLQ002957 for ; Wed, 16 Mar 2011 08:20:22 -0600 Message-ID: <4D80C724.8050408@linux.vnet.ibm.com> Date: Wed, 16 Mar 2011 07:20:20 -0700 From: "Venkateswararao Jujjuri (JV)" MIME-Version: 1.0 References: <20110315103453.GA23922@linux.vnet.ibm.com> <20110315103803.GC23922@linux.vnet.ibm.com> <4D7F65EF.8080906@us.ibm.com> <4D80B6D1.4020201@us.ibm.com> In-Reply-To: <4D80B6D1.4020201@us.ibm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: [v1 PATCH 2/3]: Helper routines to use GLib threadpool infrastructure in 9pfs. List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: aneesh.kumar@linux.vnet.ibm.com, Stefan Hajnoczi , qemu-devel@nongnu.org, arun@linux.vnet.ibm.com On 3/16/2011 6:10 AM, Anthony Liguori wrote: > On 03/16/2011 04:20 AM, Stefan Hajnoczi wrote: >> On Tue, Mar 15, 2011 at 1:13 PM, Anthony Liguori wrote: >>> Why even bothering signaling for completion with the virtio-9p threadpool? >>> >>> There's no sane guest that's going to poll on virtio-9p completion with >>> interrupts disabled and no timer. Once we enable the I/O thread by default, >>> it won't even be necessary for the paio layer. >> It's not just about preventing livelock under extreme cases. If you >> omit the signal then !CONFIG_IOTHREAD builds will see increased >> latency because virtfs request completion will piggy-back on other >> events that *do* interrupt the vcpu. > > But realistically, the timer is firing at a high enough frequency that I doubt > you'd even observe the latency. > > There's an easy solution here, just do some sniff testing to see if you can tell > the difference. I bet you can't. > >> I'm no fan of !CONFIG_IOTHREAD >> but skipping signals is currently bad practice and will continue to be >> until CONFIG_IOTHREAD is removed entirely. >> >> The proper solution would be a thin abstraction for thread-safe >> notification that compiles out signals when CONFIG_IOTHREAD is used. >> Then we have one place in QEMU that does signalling correctly and we >> can optimize it or remove CONFIG_IOTHREAD completely without having >> the burden of duplicating this code in several places. > > We have probably 5 different ways to wake up a CPU. I don't think we should add > a new one just for this. > > !CONFIG_IOTHREAD needs to go away in the very near future. I'd rather focus on > that. If that is the case, how about making VirtFS dependent on CONFIG_IOTHREAD during the configuration? This can help even if !CONFIG_IOTHREAD lingers around for some more time and would avoid any of the Stefan's concerns. - JV > > Regards, > > Anthony Liguori > >> Stefan >