From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=41767 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Pzu86-0004rm-O1 for qemu-devel@nongnu.org; Wed, 16 Mar 2011 13:03:29 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Pzu7z-000221-Gu for qemu-devel@nongnu.org; Wed, 16 Mar 2011 13:03:20 -0400 Received: from mail-gx0-f173.google.com ([209.85.161.173]:33207) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Pzu7z-00021s-EF for qemu-devel@nongnu.org; Wed, 16 Mar 2011 13:03:19 -0400 Received: by gxk26 with SMTP id 26so891158gxk.4 for ; Wed, 16 Mar 2011 10:03:19 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <4D80C724.8050408@linux.vnet.ibm.com> References: <20110315103453.GA23922@linux.vnet.ibm.com> <20110315103803.GC23922@linux.vnet.ibm.com> <4D7F65EF.8080906@us.ibm.com> <4D80B6D1.4020201@us.ibm.com> <4D80C724.8050408@linux.vnet.ibm.com> Date: Wed, 16 Mar 2011 17:03:18 +0000 Message-ID: From: Stefan Hajnoczi Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable 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: "Venkateswararao Jujjuri (JV)" Cc: arun@linux.vnet.ibm.com, Anthony Liguori , qemu-devel@nongnu.org, aneesh.kumar@linux.vnet.ibm.com On Wed, Mar 16, 2011 at 2:20 PM, Venkateswararao Jujjuri (JV) wrote: > 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 = =A0wrote: >>>> Why even bothering signaling for completion with the virtio-9p threadp= ool? >>>> >>>> There's no sane guest that's going to poll on virtio-9p completion wit= h >>>> interrupts disabled and no timer. =A0Once 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. =A0If 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. =A0I bet you can't. >> >>> =A0 =A0I'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. =A0I don't think we = should add >> a new one just for this. >> >> !CONFIG_IOTHREAD needs to go away in the very near future. =A0I'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. Sounds fair. Stefan