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Re: [fedora-virt] suggestion: a service for tuning ksm according to memory load



On Fri, Sep 04, 2009 at 06:19:29PM +0300, Dan Kenigsberg wrote:
> On Thu, Sep 03, 2009 at 01:05:47PM +0100, Daniel P. Berrange wrote:
> > On Thu, Sep 03, 2009 at 02:49:23PM +0300, Dan Kenigsberg wrote:
> > > If you are running a number of qemu-kvm's with similar guests, you can
> > > gain a lot of memory by using ksm properly.
> > > 
> > > An unattended host running a variable number of qemu-kvm's needs to tune
> > > ksm automatically, since when memory is tight, it's better to spend more
> > > cpu on merging pages. In more relaxed cases, it's just a waste of time.
> > > 
> > > The attached service tries to do just that.
> > > 
> > > It monitors how much memory is used by qemu-kvm processes, and starts
> > > ksm when a threshold is passed. Ksm usually manages to free up some
> > > memory.
> > > 
> > > As long as memory used by qemu is above the defined threshold, ksm tries
> > > harder and harder to share memory pages (up to a limit). This may happen
> > > if a guest starts working and consumes new memory. If there's enough
> > > free memory, ksm cools down.
> > > 
> > > Ksmd service has the usual start/status/stop verbs, and an additional
> > > one: signal. One should use that verb just after one starts a new
> > > qemu-kvm process or just after such process dies, to let ksm adjust
> > > immediately.
> > > 
> > > Comments and suggestion are welcome.
> > 
> > Looks like a nice idea.
> > 
> > I'd be inclined to split this file up a little to get separation of
> > the init script bits, from the tuning logic, and to allow ksm / tuning
> > to be managed more independantly, eg
> > 
> >  - /etc/init.d/ksmd     - to start/stop  ksmd
> >  - /etc/init.d/ksmtuned - to start/stop the automatic tuning process
> >  - /usr/sbin/ksmtuned   - the logic from the loop() function & things it calls
> > 
> > 
> > That makes it easier for other distros to share the important bits  - eg
> > the actual tuning logic, even if they use different initscript system
> 
> I can see the merit in splitting to /usr/sbin/ksmtuned (though who cares
> about other distros !?).
> 
> But why should we want two services? I would like ksmtuned to stop ksm
> completely when it is not needed (and start it up if needed), so what
> ksmd would be good for?

Management applications / developers might like to implement/use a
different way of tuning KSM. Thus we should be able to start the
core KSM service, without also starting the KSM tuning service.

Daniel
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