linux-kernel - Re: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to BIO

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Date:	Sun, 1 Jan 2012 16:45:42 +0000
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Chris Wright <chrisw@...s-sol.org>,
	Jens Axboe <axboe@...nel.dk>,
	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to BIO

On Thu, Dec 22, 2011 at 11:41 PM, Minchan Kim <minchan@...nel.org> wrote:
> On Thu, Dec 22, 2011 at 12:57:40PM +0000, Stefan Hajnoczi wrote:
>> On Wed, Dec 21, 2011 at 1:00 AM, Minchan Kim <minchan@...nel.org> wrote:
>> If you're stumped by the performance perhaps compare blktraces of the
>> request approach vs the bio approach.  We're probably performing I/O
>> more CPU-efficiently but the I/O pattern itself is worse.
>
> You mean I/O scheduler have many techniques to do well in I/O pattern?
> That's what I want to discuss in this RFC.
>
> I guess request layer have many techniques proved during long time
> to do well I/O but BIO-based drvier ignores them for just reducing locking
> overhead. Of course, we can add such techniques to BIO-batch driver like
> custom-batch in this series. But it needs lots of work, is really duplication,
> and will have a problem on maintenance.
>
> I would like to listen opinions whether this direction is good or bad.

This series is a good platform for performance analysis but not
something that should be merged IMO.  As you said it duplicates work
that I/O schedulers and the request-based block layer do.  If other
drivers start taking this approach too then the duplication will be
proliferated.

The value of this series is that you have a prototype to benchmark and
understand the bottlenecks in virtio-blk and the block layer better.
The results do not should that bypassing the I/O scheduler is always a
win.  The fact that you added batching suggests there is some benefit
to what the request-based code path does.  So find out what's good
about the request-based code path and how to get the best of both
worlds.

By the way, drivers for solid-state devices can set QUEUE_FLAG_NONROT
to hint that seek time optimizations may be sub-optimal.  NBD and
other virtual/pseudo device drivers set this flag.  Should virtio-blk
set it and how does it affect performance?

Stefan
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