Re: [RFC qemu 4/4] migration: filter out guest's free pages in ram bulk stage

[Cornelia Huck]

On Thu,  3 Mar 2016 18:44:28 +0800
Liang Li <liang.z.li@xxxxxxxxx> wrote:

> Get the free pages information through virtio and filter out the free
> pages in the ram bulk stage. This can significantly reduce the total
> live migration time as well as network traffic.
> 
> Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
> ---
>  migration/ram.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 46 insertions(+), 6 deletions(-)
> 

> @@ -1945,6 +1971,20 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>                                              DIRTY_MEMORY_MIGRATION);
>      }
>      memory_global_dirty_log_start();
> +
> +    if (balloon_free_pages_support() &&
> +        balloon_get_free_pages(migration_bitmap_rcu->free_pages_bmap,
> +                               &free_pages_count) == 0) {
> +        qemu_mutex_unlock_iothread();
> +        while (balloon_get_free_pages(migration_bitmap_rcu->free_pages_bmap,
> +                                      &free_pages_count) == 0) {
> +            usleep(1000);
> +        }
> +        qemu_mutex_lock_iothread();
> +
> +        filter_out_guest_free_pages(migration_bitmap_rcu->free_pages_bmap);

A general comment: Using the ballooner to get information about pages
that can be filtered out is too limited (there may be other ways to do
this; we might be able to use cmma on s390, for example), and I don't
like hardcoding to a specific method.

What about the reverse approach: Code may register a handler that
populates the free_pages_bitmap which is called during this stage?

<I like the idea of filtering in general, but I haven't looked at the
code yet>

> +    }
> +
>      migration_bitmap_sync();
>      qemu_mutex_unlock_ramlist();
>      qemu_mutex_unlock_iothread();