Re: [Qemu-devel] [PATCH v2] migration: skip sending ram pages released by virtio-balloon driver.

[Eric Blake]

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 <address@hidden>
> ---
> 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

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