[fedora-virt] disabling ksm by default

[Justin M. Forbes]

On Tue, 2009-10-27 at 15:55 +0000, Mark McLoughlin wrote:
> On Tue, 2009-10-27 at 10:46 -0500, Justin M. Forbes wrote:
> > On Tue, 2009-10-27 at 17:31 +0200, Izik Eidus wrote:
> > > On 10/27/2009 04:42 PM, Justin M. Forbes wrote:
> > >    
> > > > While I do understand what you are saying, I don't think it is worth making
> > > > a kernel change for at this point in the cycle.  Because ksm itself has a
> > > > separate initscript, people who wish to use ksm will likely turn it on.
> > > > This sets the max_kernel_pages to a reasonable value.  People who are not
> > > > interested enough to turn on the ksm service are probably not the kind of
> > > > people who will be checking to see how effective ksm is at all.
> > > >
> > > To me it sound that users have no idea about this script, and ksm merge 
> > > to him just the zero pages of windows 7...
> > > 
> > > My feeling is that 99% of the ppl in the world that will use it, would 
> > > just see the zero page merged and think "that is it..."
> > > The current behaivor in fedora 12 is misleading the user (at least it 
> > > seems to me that it misslead that specific user)
> > > 
> > > Btw we can set this value from userspace if we want "echo 0 > 
> > > /sys/kernel/mm/ksm/run"
> > 
> > Where would we put that in userspace? Another init script? I just
> > updated to documentation on the feature page (and hopefully in the final
> > release notes) to say that ksm can be enabled by running 'sudo chkconfig
> > ksm on' in effort to draw more attention to these scripts.
> > 
> > The other option is to make the ksm init script default to on, and just
> > leave ksmtuned off at system start.  Since ksm is on by the default
> > kernel, all we would be doing here is changing max_kernel_pages to a
> > machine specific value.  The ksm init script is part of the qemu
> > package, so it would only be changed on systems doing virt anyway.
> 
> For Fedora 13, it'll be off by default in the kernel and the recommended
> way of switching it on is with 'chkconfig ksm on'
> 
> For Fedora 12, it's on by default in the kernel, 'chkconfig ksm on' just
> changes max pages and the only way of disabling it is by manually
> writing zero to /sys/kernel/mm/ksm/run
> 
> IMHO, if we are to change anything for Fedora 12, we should move closer
> to Fedora 13 behaviour, not further away from it - i.e. make it off by
> default in the kernel, but I'm fine with delaying that until post GA
> 
> We'll probably end up with this behaviour in F12 updates at some point
> anyway when 2.6.32 is pulled in

This is exactly why I am more concerned with people using the init
script than changing something else.  If people use the init script,
behavior doesn't change when F12 updates to 2.6.32.  Whether or not we
default the ksm init script to on might be a conversation worth having,
but making other changes just seems a bit hackish at this point.

Justin