From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54180) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UypLD-0002Yp-3v for qemu-devel@nongnu.org; Mon, 15 Jul 2013 16:25:53 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UypLB-0008TS-Kz for qemu-devel@nongnu.org; Mon, 15 Jul 2013 16:25:51 -0400 Received: from mail.avalus.com ([2001:41c8:10:1dd::10]:34046) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UypB5-00040q-FH for qemu-devel@nongnu.org; Mon, 15 Jul 2013 16:15:23 -0400 Date: Mon, 15 Jul 2013 21:15:10 +0100 From: Alex Bligh Message-ID: <75638CBA408455BF52192DA7@Ximines.local> In-Reply-To: <51E4063D.6010308@redhat.com> References: <1373127897-3445-1-git-send-email-alex@alex.org.uk> <51E4063D.6010308@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Content-Disposition: inline Subject: Re: [Qemu-devel] [PATCH] [RFC] aio/async: Add timed bottom-halves Reply-To: Alex Bligh List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Kevin Wolf , Anthony Liguori , Stefan Hajnoczi , qemu-devel@nongnu.org, Alex Bligh , rth@twiddle.net Paolo, --On 15 July 2013 16:25:01 +0200 Paolo Bonzini wrote: Thanks for the review. > Il 06/07/2013 18:24, Alex Bligh ha scritto: >> Add timed bottom halves. A timed bottom half is a bottom half that >> will not execute until a given time has passed (qemu_bh_schedule_at) >> or a given interval has passed (qemu_bh_schedule_in). > > ... and may be delayed arbitrarily past that given interval if you are > running in qemu-img or in other synchronous I/O APIs. That's true. However, the problem with timers is worse, in that we poll for timers even less frequently as far as I can tell. > I'm especially > worried that this will not have any effect if bdrv_aio_cancel is calling > qemu_aio_wait. bdrv_aio_cancel is presumably one place where you want > timeout/reconnect functionality to trigger. Well, I'm a newbie here, so may well be wrong but I thought qemu_aio_wait /did/ call bottom halves (but didn't call QemuTimers). Provided time does actually advance (which inspection suggests it does), then these bh's should be called just like any other bh's. I may have missed the point here entirely. > I would really prefer to have a TimeEventNotifier or something like > that, which is API-compatibile with EventNotifier (so you can use the > regular aio-*.c APIs) but triggers when a given time has passed. > Basically an "heavyweight" QEMUTimer; that would be a timerfd on Linux, > and a queue timer on Windows. No idea on other POSIX systems, > unfortunately. I was trying to use the bh API because that's what the existing block drivers use, and what I really wanted was a bh that wouldn't execute for a while. Do EventNotifiers run whilst AIO is polling for completion? > Even better would be to remove the whole timer stuff (POSIX timers, > setitimer, and the Win32 equivalents), and just make the timers use a > shorter timeout for the main loop. If you do this, I suspect adding > timer support to AioContext would be much simpler. In discussion with Stefan H on IRC, I originally suggested moving the QemuTimer poll to the AIO loop (or adding another), which is a half arsed way to do what you are suggesting. He suggested this would be hairy because the existing users might not be safe to be called there. This was an attempt at a minimal change to fix that use. > BTW, note that qemu-nbd (and qemu-io too) does call timers. I'd thought I tested qemu-io. qemu-convert definitely does not. Alex > Paolo > >> Any qemu >> clock can be used, and times are specified in nanoseconds. >> >> Timed bottom halves can be used where timers cannot. For instance, >> in block drivers where there is no mainloop that calls timers >> (qemu-nbd, qemu-img), or where (per stefanha@redhat.com) the >> aio code loops internally and thus timers never get called. >> >> Signed-off-by: Alex Bligh > > -- Alex Bligh