linux-kernel - Re: [PATCH] virtio_blk: merge S/G list entries by default

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Message-ID: <20140907184745.GA23026@lst.de>
Date:	Sun, 7 Sep 2014 20:47:45 +0200
From:	Christoph Hellwig <hch@....de>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Christoph Hellwig <hch@....de>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio_blk: merge S/G list entries by default

On Sun, Sep 07, 2014 at 02:41:53PM +0300, Michael S. Tsirkin wrote:
> > Signed-off-by: Christoph Hellwig <hch@....de>
> 
> OK so this is an optimization patch right?
> What kind of performance gain is observed with it?

None.  I actually wrote it when the block layer had a bug when dm was
used on top of the !merge case, and I decided to send it out as there had been
no discussion about disabling this by default on the existing blk-mq
drivers.

At least for my qemu/kvm setup it doesn't make a difference either way,
although not doing the cheap merge doesn't like the right kind of optimization
to me.
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