[CentOS-virt] very low performance of Xen guests

Stephen John Smoogen

smooge at gmail.com
Mon Jun 15 11:46:15 UTC 2020

On Sun, 14 Jun 2020 at 14:49, Manuel Wolfshant <wolfy at nobugconsulting.ro>

> Hello
>     For the past months I've been testing upgrading my Xen hosts to CentOS
> 7 and I face an issue for which I need your help to solve.
>     The testing machines are IBM blades, model H21 and H21XM. Initial
> tests were performed on the H21 with 16 GB RAM; during the last 6=7 weeks
> I've been using the H21XM with 64 GB. In all cases the guests were fully
> updated CentOS 7 -- initially 7.6 ( most recent at the time of the initial
> tests ), and respectively 7.8 for the tests performed during the last 2
> months.  As host I used initially CentOS 6 with latest kernel available in
> the centos virt repo at the time of the tests and CentOS 7 with the latest
> kernel as well. As xen versions I tested 4.8 and 4.12 ( xl info included
> below ). The storage for the last tests is a Crucial MX500 but results were
> similar when using traditional HDD.
>     My problem, in short, is that the guests are extremely slow. For
> instance , in the most recent tests, a yum install kernel takes cca 1 min
> on the host and 12-15 (!!!) minutes in the guest, all time being spent in
> dracut regenerating the initramfs images. I've done rough tests with the
> storage  ( via dd if=/dev/zero of=a_test_file size bs=10M count=1000 ) and
> the speed was comparable between the hosts and the guests. The version of
> the kernel in use inside the guest also did not seem to make any difference
> . OTOH, sysbench ( https://github.com/akopytov/sysbench/ ) as well as
> p7zip benchmark report for the guests a speed which is between 10% and 50%
> of the host. Quite obviously, changing the elevator had no influence
> either.
>     Here is the info which I think that should be relevant for the
> software versions in use. Feel free to ask for any additional info.

Is there a way to boot up a PV guest versus an HVM? I could not find a
H21XM but found an HS21XM on the iBM site and that seemed to be a 4 core 8
thread cpu which looks 'old' enough that the Spectre/etc fixes to improve
performance after the initial hit were not done. (Basically I was told that
if the CPU was older than 2012, just turn off hyperthreading altogether to
try and get back some performance.. but don't expect much). As such I would
also try turning off HT on the CPU to see if that improves anything.

Stephen J Smoogen.
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