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

Mon Apr 11 15:46:41 UTC 2016
T.Weyergraf <T.Weyergraf at virtfinity.de>

On 04/11/2016 12:34 PM, George Dunlap wrote:
> On Sun, Apr 10, 2016 at 11:20 AM, T.Weyergraf <T.Weyergraf at virtfinity.de> wrote:
>> [...my stuff deleted...]
> 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/
Right, thanks for the link. I am aware, that the PV-part of PVHVM is 
eating some performance. Usually, this is something in the 10-20% 
ballpark, give or take the workload.
But this is more a 50% hit in walltime and close to a 5-fold jump in 
system time. Also keep in mind, that this is an E5 v3 based machine, 
with APIC-V (and Xen detecting it). I was expecting:
- that Dom0 utilizes it, however I cannot confirm it's usage
- that this will at least help with irq processing times.
> 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.
I am aware of PVH (and it's potential merits). However, as it is not 
there yet, I have no reference as to what to expect in real life. The 
point I am making, is that the performance hit observed is quite hefty, 
IMHO. I have never observed something in that magnitude before.

Note, that I might be looking at a problem, entirely unrelated to Xen.
> 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.
Yes, that is precisely what I was hoping for - other people's 
expectations/experiences. Maybe even some repeats of my simple test and 
posted results. On that machine, I use Xen as a workstation setup, where 
Dom0 also serves as my regular work environment. On the infrastructure, 
I maintain at work, Dom0 performance is next to irrelevant.
I have tried some additional things. The whole thing started with a 
4.6.0-rc2 kernel tried as Dom0 kernel, as I wanted to try the new 
multiqueue blkback/blkfront patches. Unfortunately, that did not work 
due to my graphics not being initialized properly, which I assume is 
unrelated to Xen. I'll continue poking around.
I have tried some non-Xen bare-metal kernels, including said 4.6.0-rc2, 
which yielded results consistent with the 3.18.29 being used as 
bare-metal kernel. I even applied a BIOS-update, which did not change 
things either, just to make sure (according to my experience, the 
z-series Workstations are pretty good at delivering working BIOSes, 
which is one of the main reasons for me to buy them in the first place).

I'll continue investigating and post updates, if i find anything 
noteworthy. I'd be more than happy to hear, what kinds of results others 
see and i am open to suggestions for things to try.

>   -George