Re: serious performance regression in Xen 4.2.4 and 4.2.5

[Malcolm Crossley <malcolm.crossley@citrix.com>]

On 19/02/15 14:21, Jan Beulich wrote:
>>>> On 19.02.15 at 14:08, <malcolm.crossley@citrix.com> wrote:
>> On 19/02/15 11:08, Andreas Kinzler wrote:
>>> Hello Xen developers,
>>>
>>> since we use Xen for our production systems, I run many tests on Xen
>>> (stability/performance). One test now uncovered a serious performance
>>> regression when updating from Xen 4.2.3 to 4.2.x (with x>=4). To
>>> reproduce run a domU (HVM) and compile a kernel for example ("time make
>>> -j3"). Below are my results - compilation times more than double!
>>>
>>> Is it my mistake or otherwise how could such a bug be unnoticed? Really
>>> wondering.
>>> If it is really a regression, will it be fixed since the 4.2 branch is
>>> declared unsupported?
>>>
>>
>> I believe you are being hit with an issue with uncached mappings being
>> setup when RAM is relocated out of the guest MMIO hole:
>>
>> http://lists.xen.org/archives/html/xen-devel/2015-02/msg02306.html 
>>
>> Only Intel system which do not support IOMMU snoop control are affected
>> which are typically desktop/workstation class processors.
>>
>> You have the following workarounds:
>>
>> You can configure the guest to have less than 3GB of RAM to workaround
>> the issue
>>
>> or
>>
>> Disable IOMMU support
>>
>> or
>>
>> Apply the patch I have linked to above.
> 
> I'm not really following: By default there shouldn't be any RAM
> relocation - this ought to be needed only when there are passed
> through devices with some pretty large BAR(s).
> 
That's a good point, I had forgotten about that extra condition (we've
been testing GPU passthrough and so have always had large BAR's in the
guest).

Hopefully Andreas can give us more information on the host and guest
configuration.

> Jan
>