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

[Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>]

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.

Regards

Malcolm

> Regards Andreas
> 
> Xen 4.2.3, 3.10.44-el7-pvops0-ak3, Xeon E3-1230, compile in HVM
>     real    2m58.371s
>     user    5m11.580s
>     sys     0m12.736s
> 
> Xen 4.2.4, 3.10.44-el7-pvops0-ak3, Xeon E3-1230, compile in HVM
>     real    6m44.697s    (!!!)
>     user    12m4.988s
>     sys     0m33.916s
> 
> Xen 4.2.5, 3.10.44-el7-pvops0-ak3, Xeon E3-1230, compile in HVM
>     real    6m27.998s    (!!!)
>     user    11m42.664s
>     sys     0m32.700s
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxx
> http://lists.xen.org/xen-devel


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel