[svlug] (forw) KVM vs Xen

Rick Moen rick at linuxmafia.com
Wed Oct 14 00:32:50 PDT 2015

Forwarded with permission from the main mailing list of Linux Users of
Victoria (Melbourne).

----- Forwarded message from Russell Coker <russell at coker.com.au> -----

Date: Wed, 14 Oct 2015 16:20:13 +1100
From: Russell Coker <russell at coker.com.au>
To: luv-main at luv.asn.au
Subject: KVM vs Xen

Xen has been widely regarded as the best performance VM for Linux for a long 
time.  Oracle has been one of the advocates of Xen claiming very close to 
native hardware performance.

# 300% improvement in UnixBench score, with a KVM Linode vs a Xen Linode
# 28% faster at compiling a Linux kernel with a KVM Linode vs a Xen Linode
# Boot and shutdown times are greatly improved

Now Linode (one of the largest Xen sites) is moving to KVM, they list the 
above as benefits of KVM which surprises me.  My experience of Xen is that the 
only way anything could be 300% faster is if it's an issue of disk IO 
scheduling on hard drives (as multiple virtual machines on the same spinning 
media causes contention and/or fragmentation issues depending on how you do 
it.  But given that Linode was already using SSD for all storage that's 
obviously not what they are doing.

The last time I tried KVM on my laptop the performance was a lot slower than 
native performance as opposed to Xen which was near enough to native hardware 
performance that the difference didn't matter.  I've never even tested KVM on a 
server because the performance on my laptop (admittedly a couple of years ago) 
was very disappointing.  Last time I tested KVM performance was not only 
noticably worse (EG compiles of selinux-policy-default taking about 50% 
longer) but the increase in CPU use was an issue of cooling.

Has KVM improved a lot recently?  How can anything be so much better than Xen 
when Xen has been so close to native performance for so long?

I've just chosen KVM for a new Linode instance.  They allow me to choose Xen 
but say that KVM is the way of the future - presumably I would be forced to 
use KVM sooner or later so it seemed easier to use it now.

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