Linux Virtualization - Sep 2014 - [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 at lst.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.

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 at lst.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

On Mon, Sep 08, 2014 at 11:18:30AM +0300, Michael S. Tsirkin
wrote:
> Could you respond to Ming Lei's mail, who benchmarked the patch,
please?
I don't really have any additional data or disagreement, not sure what
I should respond there.