Re: [Xen-devel] Xen 4.3 development update

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 02/05/13 16:48, Tim Deegan wrote:
> At 15:21 +0200 on 29 Apr (1367248894), Peter Maloney wrote:
> > On 04/04/2013 07:05 PM, Tim Deegan wrote:
> > > Also, if there is still a bad slowdown, caused by the p2m lookups, this
> > > might help a little bit:
> > >
> > > diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> > > index 38e87ce..7bd8646 100644
> > > --- a/xen/arch/x86/hvm/hvm.c
> > > +++ b/xen/arch/x86/hvm/hvm.c
> > > @@ -1361,6 +1361,18 @@ int hvm_hap_nested_page_fault(paddr_t gpa,
> > >           }
> > >       }
> > >   
> > > +
> > > +    /* For the benefit of 32-bit WinXP (& older Windows) on AMD CPUs,
> > > +     * a fast path for LAPIC accesses, skipping the p2m lookup. */
> > > +    if ( !nestedhvm_vcpu_in_guestmode(v)
> > > +         && gfn == vlapic_base_address(vcpu_vlapic(current)) >> PAGE_SHIFT )
> > > +    {
> > > +        if ( !handle_mmio() )
> > > +            hvm_inject_hw_exception(TRAP_gp_fault, 0);
> > > +        rc = 1;
> > > +        goto out;
> > > +    }
> > > +
> > >       p2m = p2m_get_hostp2m(v->domain);
> > >       mfn = get_gfn_type_access(p2m, gfn, &p2mt, &p2ma,
> > >                                 P2M_ALLOC | (access_w ? P2M_UNSHARE : 0), NULL);
> > This patch (applied to 4.2.2) has a very large improvement on my box
> > (AMD FX-8150) and WinXP 32 bit.
> Hmm - I expected it to be only a mild improvement.  How about this one,
> which puts in the same shortcut in another place as well?  I don't think
> it will be much better than the last one, but it's worth a try.
So I dusted off my old perf testing scripts and added in one to measure 
boot performance.
Below are boot times, from after "xl create" returns, until a specific 
python daemon running in the VM starts responding to requests.  So lower 
is better.
There are a number of places where there can be a few seconds of noise 
either way, but on the whole the tests seem fairly repeatable.
I ran this with w2k3eesp2 and with winxpsp3, using some of the 
auto-install test images made for the XenServer regression testing. All 
of them are using a flat file disk backend with qemu-traditional.
Results are in order of commits:

Xen 4.1:

w2k3: 43 34 34 33 34
winxp: 110 111 111 110 112

Xen 4.2:

w2k3: 34 44 45 45 45
winxp: 203 221 210 211 200

Xen-unstable w/ RTC fix:

w2k3: 43 44 44 45 44
winxp: 268 275 265 276 265

Xen-unstable with rtc fix + this "fast lapic" patch:

w2k3: 43 45 44 45 45
winxp: 224 232 232 232 232


So w2k3 boots fairly quickly anyway; has a 50% slow-down when moving 
from 4.1 to 4.2, and no discernible change after that.
winxp boots fairly slowly; nearly doubles in speed for 4.2, and gets 
even worse for xen-unstable.  The patch is a measurable improvement, but 
still nowhere near 4.1, or even 4.2.
On the whole however -- I'm not sure that boot time by itself is a 
blocker.  If the problem really is primarily the "eager TPR" issue for 
Windows XP, then I'm not terribly motivated either: the Citrix PV 
drivers patch Windows XP to modify the routine to be lazy (like w2k3); 
there is hardware available which allows the TPR to be virtualized; and 
there are plenty of Windows-based OSes available which do not have this 
problem.
I'll be doing some more workload-based benchmarks (probably starting 
with the Windows ddk example build) to see if there are other issues I 
turn up.
 -George

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