CentOS virt - Nov 2010 - what scheduling algorithm does KVM use?

This may not be the best place to ask, but I was prompted by a question about
guest cores on KVM.

We currently use VMWare Server (v1.0) on CentOS5.

It supports up to two virtual CPUs, but not very well, as I understand it.
VMWare Server 2.0 might do better at supporting the same maximum of 2 CPUs, but
if my research is correct, they both use what is called "strict
co-scheduling".
 Which means, if a two virtual-CPU VM is waiting for a time slice on the
physical host, there needs to be two physical CPUs available to be allocated.
This is because amount of time scheduled to each virtual CPU has to stay equal,
else assumptions in the virtualised software may become false and things start
going wrong.

The problem with this is that it is obviously more of a constraint to have two
physical CPUs available.  Therefore adding virtual CPUs to a co-scheduled VM can
actually make performance worse if the physical CPUs are under any sort of
contention.  Performance degrades rapidly under load - which is exactly what you
don't want for a webserver.  Therefore advice seems to be to avoid using
multi-virtual CPU VMs.  [1]


VMWare ESX uses something supposedly better, called relaxed co-scheduling. [2].
We're not likely to use that any time soon, since we are using 1&1 root
servers
running CentOS.

I did try and research the algorithm(s) used by KVM. Apparently it doesn't
use
co-scheduling, but possibly something called the "completely fair
scheduler",
but I'm no an expert in VM scheduling and I didn't manage to discover
what this
means, nor specifically what the implications are for performance were in terms
of scaling up the number of virtual CPUs on KVM.  (I understand that the number
of virtual CPUs supported by KVM is be quite high.)


Anyway, I've been asked how to scale a VM up utilise more - say four or more
-
physical CPUs.  Is KVM better at this than VMware Server, or does the same basic
problem persist?

Thanks,

Nick


1. http://communities.vmware.com/thread/169323

  
http://cs.gmu.edu/~hfoxwell/cs671projects/southern_v12n.pdf
> "The SMP
> scheduling algorithms used by ESX server and by Xen
> have a low overhead for single-threaded compute inten-
> sive workloads. However, they do not scale as well for
> multi-threaded workloads on a system with overcommit-
> ted CPU resources."

2. http://communities.vmware.com/thread/157849
   http://communities.vmware.com/docs/DOC-4960
   http://communities.vmware.com/docs/DOC-5501

2010/11/19 Nick <oinksocket at letterboxes.org>:
> The problem with this is that it is obviously more of a constraint to have
two
> physical CPUs available. ?Therefore adding virtual CPUs to a co-scheduled
VM can
> actually make performance worse if the physical CPUs are under any sort of
> contention. ?Performance degrades rapidly under load - which is exactly
what you
> don't want for a webserver. ?Therefore advice seems to be to avoid
using
> multi-virtual CPU VMs. ?[1]
As long as you keep the number of CPUs for *each* VM - equal to or
lower than - the number of physical cores, your performance should not
suffer with KVM. If you assign more CPUs to one VM than you have
available cores (a core can also be a hyperthreaded core), you will
for sure run into performance problems.
> I did try and research the algorithm(s) used by KVM. Apparently it
doesn't use
> co-scheduling, but possibly something called the "completely fair
scheduler",
The good thing about KVM compared to other virtualization solutions,
is that KVM doesn't try to reinvent the wheel. It leaves scheduling to
the Linux kernel, so whatever your Linux system is setup to use, KVM
will use that. You can choose to run CFS (Completely Fair Scheduler),
deadline, BFS, or whatever scheduler you prefer. As long as Linux uses
it, KVM will use it.
> but I'm no an expert in VM scheduling and I didn't manage to
discover what this
> means, nor specifically what the implications are for performance were in
terms
> of scaling up the number of virtual CPUs on KVM. ?(I understand that the
number
> of virtual CPUs supported by KVM is be quite high.)
If you want a really competent answer to your question, send it to the
kvm-devel list...they don't mind user questions like this, and as long
as you're asking a relevant question, there's a good chance of getting
a highly competent answer :)

Best regards
Kenni