On 03/27/2016 10:16 PM, Jitendra Kolhe wrote: > While measuring live migration performance for qemu/kvm guest, it > was observed that the qemu doesn’t maintain any intelligence for the > guest ram pages which are released by the guest balloon driver and > treat such pages as any other normal guest ram pages. This has direct > impact on overall migration time for the guest which has released > (ballooned out) memory to the host. > > In case of large systems, where we can configure large guests with 1TB > and with considerable amount of memory release by balloon driver to the, > host the migration time gets worse.
s/the, host/the host,/
>
> The optimization gets temporarily disabled, if the balloon operation is
s/disabled,/disabled/
> in progress. Since the optimization skips scanning and migrating control
> information for ballooned out pages, we might skip guest ram pages in
> cases where the guest balloon driver has freed the ram page to the guest
> but not yet informed the host/qemu about the ram page
> (VIRTIO_BALLOON_F_MUST_TELL_HOST). In such case with optimization, we
> might skip migrating ram pages which the guest is using. Since this
> problem is specific to balloon leak, we can restrict balloon operation in
> progress check to only balloon leak operation in progress check.
>
> The optimization also get permanently disabled (for all subsequent
s/get/gets/
> migrations) in case any of the migration uses postcopy capability. In case
> of postcopy the balloon bitmap would be required to send after vm_stop,
> which has significant impact on the downtime. Moreover, the applications
> in the guest space won’t be actually faulting on the ram pages which are
> already ballooned out, the proposed optimization will not show any
> improvement in migration time during postcopy.
>
> Signed-off-by: Jitendra Kolhe <jitendra.ko...@hpe.com>
> ---
> Changed in v2:
> - Resolved compilation issue for qemu-user binaries in exec.c
> - Localize balloon bitmap test to save_zero_page().
> - Updated version string for newly added migration capability to 2.7.
> - Made minor modifications to patch commit text.
I'll leave the technical review to others.
> +++ b/qapi-schema.json
> @@ -544,11 +544,14 @@
> # been migrated, pulling the remaining pages along as needed. NOTE:
> If
> # the migration fails during postcopy the VM will fail. (since 2.6)
> #
> +# @skip-balloon: Skip scanning ram pages released by virtio-balloon driver.
> +# (since 2.7)
> +#
> # Since: 1.2
> ##
> { 'enum': 'MigrationCapability',
> 'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
> - 'compress', 'events', 'postcopy-ram'] }
> + 'compress', 'events', 'postcopy-ram', 'skip-balloon'] }
Does this flag make sense to always have enabled (in which case we don't
need it as a flag), or are there cases where we'd explicitly want to
disable it?
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
signature.asc
Description: OpenPGP digital signature
