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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 11:18, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> And second, I have been fighting with finding both conditions
> and (eventually) the root cause of a severe performance
> regression (compared to 4.3.x) I'm observing on an EPT+IOMMU
> system. This became _much_ worse after adding in the patch here
> (while in fact I had hoped it might help with the originally observed
> degradation): X startup fails due to timing out, and booting the
> guest now takes about 20 minutes). I didn't find the root cause of
> this yet, but meanwhile I know that
> - the same isn't observable on SVM
> - there's no problem when forcing the domain to use shadow
>   mode
> - there's no need for any device to actually be assigned to the
>   guest
> - the regression is very likely purely graphics related (based on
>   the observation that when running something that regularly but
>   not heavily updates the screen with X up, the guest consumes a
>   full CPU's worth of processing power, yet when that updating
>   doesn't happen, CPU consumption goes down, and it goes further
>   down when shutting down X altogether - at least as log as the
>   patch here doesn't get involved).
> 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.

While I'll try to look into this further, I guess I have to defer to our
VT-d specialists at Intel at this point...


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