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

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Message-ID: <20140908081830.GA32268@redhat.com>
Date:	Mon, 8 Sep 2014 11:18:30 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Ming Lei <tom.leiming@...il.com>
Subject: Re: [PATCH] virtio_blk: merge S/G list entries by default

On Sun, Sep 07, 2014 at 08:47:45PM +0200, Christoph Hellwig wrote:
> 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.

Could you respond to Ming Lei's mail, who benchmarked the patch, please?

-- 
MST
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