Re: [Xen-users] SOLVED - Poor Windows 2003 + GPLPV performance compared to VMWare

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2012-09-19 at 14:00 +0100, James Harper wrote:
> > 
> > On Wed, 2012-09-19 at 13:19 +0100, Adam Goryachev wrote:
> > > On 19/09/12 19:43, Adam Goryachev wrote:
> > > > I've updated to SP2, and will re-test in the next few hours,
> > >
> > > Excellent news, the issue is completely resolved, and in fact,
> > > performance is now better than vmware, and better than the old
> > > original physical box pre-virtualisation. The entire task now
> > > completed in 30 minutes, the previous best was 40 minutes (ie, prior to
> > xen).
> > 
> > Excellent news!
> > 
> > James, since you suggested it, do you happen to know what it is about
> > pre-SP2 W2K3 that is so bad? Do those versions beat on the TPR or
> > something else?
> > 
> > (Just curious)
> > 
> 
> Yes it will almost certainly be TPR access. I actually thought MS made
> the change in SP1 but maybe it was SP2 (don't know if the OP upgraded
> from RTM to sp2 or sp1 to sp2, but quite possibly I just don't
> remember :)
> 
> The change they made means that the TPR doesn't get touched at all
> anymore, so is much faster.
> 
> For prior versions (and XP and 2000), GPLPV can patch AMD systems to
> use the CR8 (I think) registr for TPR access which is much faster. For
> Intel, the best I could do was cache TPR so that reads were fast...
> it's still quite a speedup. I thought Xen optimised this for Intel
> though and my patching wasn't necessary anymore? Or maybe the OP is
> running a version of xen that doesn't have that feature??

I think there was a h/w feature introduced at some point on both AMD and
Intel which also optimised these vmexits away. I'm not 100% sure of the
specifics though.

Ian


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