Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Anthony Liguori]

On 05/18/2010 11:26 AM, Alexander Graf wrote:
> Jes Sorensen wrote:
>    
> > On 05/14/10 23:52, Stefan Hajnoczi wrote:
> >
> >      
> > > The VirtIOBlockRequest structure is about 40 KB in size.  This patch
> > > avoids zeroing every request by only initializing fields that are read.
> > > The other fields are either written to or may not be used at all.
> > >
> > > Oprofile shows about 10% of CPU samples in memset called by
> > > virtio_blk_alloc_request().  The workload is
> > > dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
> > > times.  This patch makes memset disappear to the bottom of the profile.
> > >
> > > Signed-off-by: Stefan Hajnoczi<address@hidden>
> > >
> > >        
> > Great catch!
> >
> > I ran some benchmarks using a ramdisk passed to the guest as a virtio
> > device and with this patch I saw improvements ranging from 5-20%. I
> > believe the fluctuations are due to not being able to numa bind it due
> > to limited memory.
> >
> > However a win all the way round!
> >
> >      
> It looks like a fairly small change with a huge win. Sounds like a
> perfect candidate for 0.12.5 to me.
>    

I'd prefer to stick to bug fixes for stable releases.  Performance 
improvements are a good motivation for people to upgrade to 0.13 :-)

Regards,

Anthony Liguori

> Alex
>
>
>