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

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Jun 13, 2016 at 11:50:08AM +0100, Daniel P. Berrange wrote:
> On Mon, Jun 13, 2016 at 06:16:45PM +0800, Liang Li wrote:
> > Add the hmp and qmp interface to drop vm's page cache, users
> > can control the type of cache they want vm to drop.
> > 
> > Signed-off-by: Liang Li <liang.z.li@intel.com>
> > ---
> >  balloon.c        | 19 +++++++++++++++++++
> >  hmp-commands.hx  | 15 +++++++++++++++
> >  hmp.c            | 22 ++++++++++++++++++++++
> >  hmp.h            |  3 +++
> >  monitor.c        | 18 ++++++++++++++++++
> >  qapi-schema.json | 35 +++++++++++++++++++++++++++++++++++
> >  qmp-commands.hx  | 23 +++++++++++++++++++++++
> >  7 files changed, 135 insertions(+)
> 
> > diff --git a/qapi-schema.json b/qapi-schema.json
> > index 8483bdf..117f70a 100644
> > --- a/qapi-schema.json
> > +++ b/qapi-schema.json
> > @@ -1655,6 +1655,41 @@
> >  { 'command': 'balloon', 'data': {'value': 'int'} }
> >  
> >  ##
> > +# @DropCacheType
> > +#
> > +# Cache types enumeration
> > +#
> > +# @clean: Drop the clean page cache.
> > +#
> > +# @slab: Drop the slab cache.
> > +#
> > +# @all: Drop both the clean and the slab cache.
> > +#
> > +# Since: 2.7
> > +##
> > +{ 'enum': 'DropCacheType', 'data': ['clean', 'slab', 'all'] }
> 
> Presumably these constants are corresponding to the 3 options
> for vm.drop_caches sysctl knob
> 
> [quote]
> To free pagecache, use:
> 
>   echo 1 > /proc/sys/vm/drop_caches
> 
> To free dentries and inodes, use:
> 
>   echo 2 > /proc/sys/vm/drop_caches
> 
> To free pagecache, dentries and inodes, use:
> 
>   echo 3 > /proc/sys/vm/drop_caches
> 
> 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 ?
> 
> > +
> > +##
> > +# @balloon_drop_cache:
> > +#
> > +# Request the vm to drop its cache.
> > +#
> > +# @value: the type of cache want vm to drop
> > +#
> > +# Returns: Nothing on success
> > +#          If the balloon driver is enabled but not functional because the KVM
> > +#            kernel module cannot support it, KvmMissingCap
> > +#          If no balloon device is present, DeviceNotActive
> > +#
> > +# Notes: This command just issues a request to the guest.  When it returns,
> > +#        the drop cache operation may not have completed.  A guest can drop its
> > +#        cache independent of this command.
> > +#
> > +# Since: 2.7.0
> > +##
> > +{ '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 defined at all useful
> to those other OS ?
> 
> 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...

Also, I'm wondering about the overall performance benefit of dropping
guest cache(s). Increasing the amount of free memory pages may have
a benefit in terms of reducing data that needs to be migrated, but
it comes with a penalty that if the guest OS needs that data, it will
have to repopulate the caches.

If the guest is merely reading those cached pages, it isn't going to
cause any problem with chances of convergance of migration, as clean
pages will be copied only once during migration. IOW, dropping clean
pages will reduce the total memory that needs to be copied, but won't
have notable affect on convergance of live migration. Cache pages that
are dirty will potentially affect live migration convergance, if the
guest OS re-dirties the pages before they're flushed to storage. Dropping
caches won't help in this respect though, since you can't drop dirty
pages. At the same time it will have a potentially significant negative
penalty on guest OS performance by forcing the guest to re-populate the
cache from slow underlying storage.  I don't think there's enough info
exposed by KVM about the guest OS to be able to figure out what kind of
situation we're in wrt the guest OS cache usage.

Based on this I think it is hard to see how a host mgmt app can make a
well informed decision about whether telling the guest OS to drop caches
is a positive thing overall. In fact I think most likely is that a mgmt
app would take a pessimistic view and not use this functionality, because
there's no clearly positive impact on migration convergance and high
liklihood of negatively impacting guest performance.

Regards,
Daniel
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