Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining()

[Peter Xu <peterx@redhat.com>]

On Sun, Apr 01, 2018 at 12:25:36AM +0530, Balamuruhan S wrote:
> expected_downtime value is not accurate with dirty_pages_rate * page_size,
> using ram_bytes_remaining would yeild it correct.
> 
> Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
> ---
>  migration/migration.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 58bd382730..4e43dc4f92 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2245,8 +2245,7 @@ static void migration_update_counters(MigrationState *s,
>       * recalculate. 10000 is a small enough number for our purposes
>       */
>      if (ram_counters.dirty_pages_rate && transferred > 10000) {
> -        s->expected_downtime = ram_counters.dirty_pages_rate *
> -            qemu_target_page_size() / bandwidth;
> +        s->expected_downtime = ram_bytes_remaining() / bandwidth;

This field was removed in e4ed1541ac ("savevm: New save live migration
method: pending", 2012-12-20), in which remaing RAM was used.

And it was added back in 90f8ae724a ("migration: calculate
expected_downtime", 2013-02-22), in which dirty rate was used.

However I didn't find a clue on why we changed from using remaining
RAM to using dirty rate...  So I'll leave this question to Juan.

Besides, I'm a bit confused on when we'll want such a value.  AFAIU
precopy is mostly used by setting up the target downtime before hand,
so we should already know the downtime before hand.  Then why we want
to observe such a thing?

Thanks,

-- 
Peter Xu