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

Jan Beulich wrote on 2014-02-17:
>>>> 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...

Hi, Jan,

I tried to reproduce it. But unfortunately, I cannot reproduce it in my box 
(sandy bridge EP)with latest Xen(include my patch). I guess my configuration or 
steps may wrong, here is mine:

1. add iommu=1,no-snoop in by xen cmd line:
(XEN) Intel VT-d Snoop Control not enabled.
(XEN) Intel VT-d Dom0 DMA Passthrough not enabled.
(XEN) Intel VT-d Queued Invalidation enabled.
(XEN) Intel VT-d Interrupt Remapping enabled.
(XEN) Intel VT-d Shared EPT tables enabled.

2. boot a rhel6u4 guest.

3. after guest boot up, run startx inside guest.

4. a few second, the X windows shows and didn't see any error. Also the CPU 
utilization is about 1.7%.

Any thing wrong?

> Jan

Best regards,

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