Re: [Qemu-devel] [QEMU 3/7] Add the hmp and qmp interface for dropping cache

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Li, Liang Z (liang.z.li@intel.com) wrote:
> > Because writing to this file is a nondestructive operation and dirty objects are
> > not freeable, the user should run sync(1) first.
> > [/quote]
> > 
> > IOW, by 'slab' you mean dentries and inodes ?
> > 
> Yes.
> 
> > > +##
> > > +{ 'command': 'balloon_drop_cache', 'data': {'value': 'DropCacheType'}
> > > +}
> > 
> > Also, as noted in the man page quote above, it is recommended to call
> > sync() to minimise dirty pages. Should we have a way to request a sync as
> > part of this monitor command.
> > 
> > More generally, it feels like this is taking as down a path towards actively
> > managing the guest kernel VM from the host. Is this really a path we want to
> > be going down, given that its going to take us into increasing non-portable
> > concepts which are potentially different for each guest OS kernel.  Is this
> > drop caches feature at all applicable to Windows, OS-X, *BSD guest OS impls
> > of the balloon driver ? If it is applicable, are the 3 fixed constants you've
> 
> No. 
> 
> > defined at all useful to those other OS ?
> > 
> 
> Maybe they are not.  
> I agree that there are too Linux specific.  And I did more than needed.
> Actually, I just want to drop the clean cache, do more than that is too heavy
> and no good for performance.
> 
> > I'm warying of us taking a design path which is so Linux specific it isn't useful
> > elsewhere. IOW, just because we can do this, doesn't mean we should do
> > this...
> > 
> 
> Agree.

I can see an argument for giving the guest a hint about what's going on and letting
the guest decide what it's going to do - so telling the guest that a migration
is happening and you'd like it to make the hosts life easy seems reasonable
and it doesn't make any guest OS assumptions.

Dave

> 
> Thanks!
> 
> Liang
> > Regards,
> > Daniel
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK