How many threads should a kvm vm be starting?

How many threads should a kvm vm be starting?

[Thomas Fjellstrom]

I just noticed something interesting, a virtual machine on one of my servers 
seems to have 69 threads (including the main thread). Other guests on the 
machine only have a couple threads.

Is this normal? or has something gone horribly wrong?

-- 
Thomas Fjellstrom
thomas@fjellstrom.ca

Re: How many threads should a kvm vm be starting?

[Avi Kivity]

On 09/27/2011 03:29 AM, Thomas Fjellstrom wrote:
> I just noticed something interesting, a virtual machine on one of my servers
> seems to have 69 threads (including the main thread). Other guests on the
> machine only have a couple threads.
>
> Is this normal? or has something gone horribly wrong?
>

It's normal if the guest does a lot of I/O.  The thread count should go 
down when the guest idles.

-- 
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Re: How many threads should a kvm vm be starting?

[Thomas Fjellstrom]

On September 27, 2011, Avi Kivity wrote:
> On 09/27/2011 03:29 AM, Thomas Fjellstrom wrote:
> > I just noticed something interesting, a virtual machine on one of my
> > servers seems to have 69 threads (including the main thread). Other
> > guests on the machine only have a couple threads.
> > 
> > Is this normal? or has something gone horribly wrong?
> 
> It's normal if the guest does a lot of I/O.  The thread count should go
> down when the guest idles.

Ah, that would make sense. Though it kind of defeats assigning a vm a single 
cpu/core. A single VM can now DOS an entire multi-core-cpu server. It pretty 
much pegged my dual core (with HT) server for a couple hours.

-- 
Thomas Fjellstrom
thomas@fjellstrom.ca

Re: How many threads should a kvm vm be starting?

[Daniel P. Berrange]

On Tue, Sep 27, 2011 at 04:04:41PM -0600, Thomas Fjellstrom wrote:
> On September 27, 2011, Avi Kivity wrote:
> > On 09/27/2011 03:29 AM, Thomas Fjellstrom wrote:
> > > I just noticed something interesting, a virtual machine on one of my
> > > servers seems to have 69 threads (including the main thread). Other
> > > guests on the machine only have a couple threads.
> > > 
> > > Is this normal? or has something gone horribly wrong?
> > 
> > It's normal if the guest does a lot of I/O.  The thread count should go
> > down when the guest idles.
> 
> Ah, that would make sense. Though it kind of defeats assigning a vm a single 
> cpu/core. A single VM can now DOS an entire multi-core-cpu server. It pretty 
> much pegged my dual core (with HT) server for a couple hours.

You can mitigate these problems by putting each KVM process in its own
cgroup, and using the 'cpu_shares' tunable to ensure that each KVM
process gets the same relative ratio of CPU time, regardless of how
many threads it is running. With newer kernels there are other CPU
tunables for placing hard caps on CPU utilization of the process as
a whole too.

Regards,
Daniel
-- 
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Re: How many threads should a kvm vm be starting?

[Thomas Fjellstrom]

On September 28, 2011, Daniel P. Berrange wrote:
> On Tue, Sep 27, 2011 at 04:04:41PM -0600, Thomas Fjellstrom wrote:
> > On September 27, 2011, Avi Kivity wrote:
> > > On 09/27/2011 03:29 AM, Thomas Fjellstrom wrote:
> > > > I just noticed something interesting, a virtual machine on one of my
> > > > servers seems to have 69 threads (including the main thread). Other
> > > > guests on the machine only have a couple threads.
> > > > 
> > > > Is this normal? or has something gone horribly wrong?
> > > 
> > > It's normal if the guest does a lot of I/O.  The thread count should go
> > > down when the guest idles.
> > 
> > Ah, that would make sense. Though it kind of defeats assigning a vm a
> > single cpu/core. A single VM can now DOS an entire multi-core-cpu
> > server. It pretty much pegged my dual core (with HT) server for a couple
> > hours.
> 
> You can mitigate these problems by putting each KVM process in its own
> cgroup, and using the 'cpu_shares' tunable to ensure that each KVM
> process gets the same relative ratio of CPU time, regardless of how
> many threads it is running. With newer kernels there are other CPU
> tunables for placing hard caps on CPU utilization of the process as
> a whole too.

I'll have to look into how to set that up with libvirt. A brief search leads 
me to believe its rather easy to set up, so I'll have to do that asap :)

> Regards,
> Daniel


-- 
Thomas Fjellstrom
thomas@fjellstrom.ca