[CentOS-virt] CentOS7, Xen 4.6.1, kernel 3.18.25-19 strange performance problem

[George Dunlap]

On Sun, Apr 10, 2016 at 11:20 AM, T.Weyergraf <T.Weyergraf at virtfinity.de> wrote:
> Hi all,
>
> i just stumbled over a strange performance issue with my Xen setup.
>
> I use centos-virt Xen since a long time on my workstation and usually never
> check performance. However, yesterday I booted into the 3.18.25-19 Dom0
> kernel *without* Xen and found the system noticably more responsive. That
> triggered me into running a simple kernel compile benchmark and compare the
> results:

Thanks for the report.  64-bit PV is known to be slow for Xen for
workloads that involve a lot of system calls (which kernel build
certainly will).  You can read an explanation for the issue, and our
intended solution, in this blog post I wrote a few years ago:

https://blog.xenproject.org/2012/10/31/the-paravirtualization-spectrum-part-2-from-poles-to-a-spectrum/

Unfortunately the initial implementation of PVH turned out to have
some technical issues, and it's in the process of being revamped.  The
work is actively ongoing, but it's likely to be another year at least
before we're going to be able to default to a PVH dom0 kernel for
CentOS.

Still, over 3x for a kernel build is rather surprising -- I'll ask
around and see what other people's expectations / experiences are.
It's possible there's a regression in the CentOS build that hasn't
been noticed.  In particular, if you happen to find any other
combination of Xen / kernel which is significantly faster, do report
it so we can look into it.

 -George