Re: how to tweak kernel to get the best out of kvm?

[Avi Kivity <avi@redhat.com>]

On 03/05/2010 05:20 PM, Harald Dunkel wrote:
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> Hi folks,
>
> Problem: My kvm server (8 cores, 64 GByte RAM, amd64) can eat up
> all block device or file system performance, so that the kvm clients
> become almost unresponsive. This is _very_ bad. I would like to make
> sure that the kvm clients do not affect each other, and that all
> (including the server itself) get a fair part of computing power and
> memory space.
>    

Please describe the issue in detail, provide output from 'vmstat' and 'top'.

> What config options would you suggest to build and run a Linux
> kernel optimized for running kvm clients?
>
> Sorry for asking, but AFAICS some general guidelines for kvm are
> missing here. Of course I saw a lot of options in Documentation/\
> kernel-parameters.txt, but unfortunately I am not a kernel hacker.
>
> Any helpful comment would be highly appreciated.
>    

One way to ensure guests don't affect each other is not to overcommit, 
that is make sure each guest gets its own cores, there is enough memory 
for all guests, and guests have separate disks.  Of course that defeats 
the some of the reasons for virtualizing in the first place; but if you 
share resources, some compromises must be made.

If you do share resources, then Linux manages how they are shared.  The 
scheduler will share the processors, the memory management subsystem 
will share memory, and the I/O scheduler will share disk bandwidth.  If 
you see a problem in one of these areas you will need to tune the 
subsystem that is misbehaving.

There is also a larger effort to improve control of sharing called 
control groups (cgroups).  You may want to read up on this as it can 
provide very fine grain control on resource sharing.

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