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Re: [Xen-devel] [PATCH] Don't track all memory when enabling log dirty to track vram

>>> On 17.02.14 at 16:00, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>>>> On 17.02.14 at 11:18, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>> This I'm observing on a Westmere box (and I didn't notice it earlier
>> because that's one of those where due to a chipset erratum the
>> IOMMU gets turned off by default), so it's possible that this can't
>> be seen on more modern hardware. I'll hopefully find time today to
>> check this on the one newer (Sandy Bridge) box I have.
> Just got done with trying this: By default, things work fine there.
> As soon as I use "iommu=no-snoop", things go bad (even worse
> than one the older box - the guest is consuming about 2.5 CPUs
> worth of processing power _without_ the patch here in use, so I
> don't even want to think about trying it there); I guessed that to
> be another of the potential sources of the problem since on that
> older box the respective hardware feature is unavailable.

That wasn't a fair comparison: The guest here had an SR-IOV NIC
assigned. Once removing that, badness goes back to about the
level I observe on the Westmere box. I'm then relatively certain
that this "extra" badness can be attributed to the excessive use of
wbinvd. Which of course puts under question what good assigning
a device does on a system without snoop control, if overall
performance gets worse rather than better.


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