Re: [Xen-devel] Re: poor domU VBD performance.

[Andrew Theurer <habanero@xxxxxxxxxx>]

peter bier wrote:

> Ian Pratt <m+Ian.Pratt <at> cl.cam.ac.uk> writes:
>
>  
>
> > > I'll check the xen block driver to see if there's anything 
> > > else that sticks out.
> > > Jens Axboe
> > >      
> > Jens, I'd really appreciate this.
> >
> > The blkfront/blkback drivers have rather evolved over time, and I don't
> > think any of the core team fully understand the block-layer differences
> > between 2.4 and 2.6. 
> >
> > There's also some junk left in there from when the backend was in Xen
> > itself back in the days of 1.2, though Vincent has prepared a patch to
> > clean this up and also make 'refreshing' of vbd's work (for size
> > changes), and also allow the blkfront driver to import whole disks
> > rather than paritions. We had this functionality on 2.4, but lost it in
> > the move to 2.6.
> >
> > My bet is that it's the 2.6 backend that is where the true perofrmance
> > bug lies. Using a 2.6 domU blkfront talking to a 2.4 dom0 blkback seems
> > to give good performance under a wide variety of circumstances. Using a
> > 2.6 dom0 is far more pernickety. I agree with Andrew that I suspect it's
> > the work queue changes are biting us when we don't have many outstanding
> > requests.
> >
> > Thanks,
> > Ian
> >
> >    
>
> I have done my simple dd on hde1 with two different setting of readahead:
> 256 sectors and 512 sectors.
I added a counter and incremented every time blkback daemon was woken up 
and ran the read test in domU.  With 32k and 320k request sizes 
(o_direct), I consistently got 200 wake ups/second.  I expected 
100/second, the same interval as the minimum svc cmt times I am seeing, 
but anyway, 200/sec is way to low for small request sizes.  I think this 
confirms the latency issue.  Not sure yet why it cannot wake up more 
frequently.
-Andrew


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