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Re: [Xen-devel] serious performance regression in Xen 4.2.4 and 4.2.5

>>> On 19.02.15 at 14:08, <malcolm.crossley@xxxxxxxxxx> 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).


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