From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:44591)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <paolo.bonzini@gmail.com>) id 1X1ZTQ-0000LX-9g
	for qemu-devel@nongnu.org; Mon, 30 Jun 2014 07:10:21 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <paolo.bonzini@gmail.com>) id 1X1ZTH-0005zF-8X
	for qemu-devel@nongnu.org; Mon, 30 Jun 2014 07:10:12 -0400
Received: from mail-wg0-x232.google.com ([2a00:1450:400c:c00::232]:62876)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <paolo.bonzini@gmail.com>) id 1X1ZTH-0005ws-1T
	for qemu-devel@nongnu.org; Mon, 30 Jun 2014 07:10:03 -0400
Received: by mail-wg0-f50.google.com with SMTP id m15so7767417wgh.33
	for <qemu-devel@nongnu.org>; Mon, 30 Jun 2014 04:10:02 -0700 (PDT)
Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <53B14588.5040807@redhat.com>
Date: Mon, 30 Jun 2014 13:10:00 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1404121756-14544-1-git-send-email-ming.lei@canonical.com>
In-Reply-To: <1404121756-14544-1-git-send-email-ming.lei@canonical.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH 0/3] block: introduce submit I/O at batch
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Ming Lei <ming.lei@canonical.com>, Peter Maydell <peter.maydell@linaro.org>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>

Il 30/06/2014 11:49, Ming Lei ha scritto:
> Hi,
>
> The commit 580b6b2aa2(dataplane: use the QEMU block layer for I/O)
> introduces ~40% throughput regression on virtio-blk dataplane, and
> one of causes is that submitting I/O at batch is removed.
>
> This patchset trys to introduce this mechanism on block, at least,
> linux-aio can benefit from that.
>
> With these patches, it is observed that thoughout on virtio-blk
> dataplane can be improved a lot, see data in commit log of patch
> 3/3.
>
> It should be possible to apply the batch mechanism to other devices
> (such as virtio-scsi) too.

The basic idea of the code is great, however I do not think it is 
necessary to add the logic in AioContext.  You are right that io_submit 
supports multiple I/O, but right now we have one io_context_t per 
BlockDriverState so it is somewhat premature.  Note that AioContext in 
QEMU means something else than io_context_t.

I don't think it's necessary to walk backing_hd too (especially because 
we're close to release and this cannot be a regression---dataplane in 
2.0 didn't support backing files at all!).  We need to keep the patches 
as simple as possible.

You can just add bdrv_plug/bdrv_unplug callbacks to block/linux-aio.c 
and, in block.c, something like

     BlockDriver *drv = bs->drv;
     if (drv && drv->bdrv_plug) {
         drv->bdrv_plug(bdrv);
     } else if (bs->file) {
         bdrv_plug(bs->file);
     }

and place all the queuing logic in linux-aio.c.  It should be obvious to 
the reviewer that the patch only affects dataplane.

(Also, note that QEMU doesn't use __ as the prefix for internal function).

Thanks for the patch!

Paolo