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 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...

Regards,
Daniel
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