[CentOS-virt] very low performance of Xen guests

Thu Jun 18 00:15:35 UTC 2020
Manuel Wolfshant <wolfy at nobugconsulting.ro>

On 6/15/20 5:40 PM, Stephen John Smoogen wrote:
> On Mon, 15 Jun 2020 at 09:42, Manuel Wolfshant 
> <wolfy at nobugconsulting.ro <mailto:wolfy at nobugconsulting.ro>> wrote:
>     On 6/15/20 2:46 PM, Stephen John Smoogen wrote:
>     I got inspired by Adi's earlier suggestion and after reading
>     https://access.redhat.com/articles/3311301 I've tried today all
>     variants of disabling the spectre mitigations. Whatever I do,
>     immediately after a reboot, yum reinstall kernel does not take
>     less than 5 minutes :( It goes down to 2 min if I repeat the
>     operation afterwards so I guess some caching kicks in. I will try
>     later today the kernels from elrepo and maybe even xen.crc.id.au
>     <http://xen.crc.id.au> ( I kind of hate the "disable selinux"
>     recommendation from the install page so I postponed it in the hope
>     of other solution ).
> If you can do a full reinstall, could you see if a KVM host/guest 
> combo has the same problem? That would at least point the finger more 
> firmly at VT, spectre or something else.

I finally managed to install a fresh KVM host / guest pair on an 
identical blade ( HS21XM, 64 GB ram, 2*E5450@ 3.00GHz ). Here are the 
results I see:

1. KVM host, stock instalation and fully updated, kernel 3.10.0-1127.10.1
#cd /sys/kernel/debug/x86/
#cat ibrs_enabled pti_enabled retp_enabled

#time yum -y reinstall kernel-3.10.0-1127.el7.x86_64
real    0m50.026s
user    0m32.872s
sys     0m23.312s

2. KVM guest on the same machine (virt-install --name guest1-rhel7 
--memory 2048 --vcpus 2  --disk size=20 --network=bridge:br0 --pxe 
--os-variant rhel7 <=== copy/paste from 
), stock installation and fully updated with absolutely no change 
towards the defaults including same ibrs_enabled pti_enabled 
retp_enabled as the host, , kernel 3.10.0-1127.10.1

#time yum -y reinstall kernel-3.10.0-1127.el7.x86_64
real    2m39.644s
user    1m54.662s
sys     1m32.496s

3. Xen Domu,  3.10.0-1127.8.2.el7.x86_6 ( but results are consistent 
across all kernels )

# cat ibrs_enabled pti_enabled retp_enabled

# time yum -y reinstall kernel-3.10.0-1127.el7.x86_64
real    5m44.030s
user    2m9.931s
sys     4m7.771s

4. Dom0, 4.9.215-36.el7.x86_64, , xen 4.12 from centos' repo

# time  yum -y reinstall kernel-3.10.0-1127.el7.x86_64
real    1m52.417s
user    0m45.704s
sys     1m32.167s

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