Peter Xu <pet...@redhat.com> wrote: > 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 <bal...@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.
Unrelated O:-) > And it was added back in 90f8ae724a ("migration: calculate > expected_downtime", 2013-02-22), in which dirty rate was used. We didn't want to update the field if there haven't been enough activity. > 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? What that field means is how much time the system needs to send everything that is pending. I.e. if expected_downtime = 2seconds, it means that with current dirty rate, if we set a downtime of 2 or bigger it is going to finish migration. It is a help for upper layers to decide that: - they want a 1second downtime - system calculates with current load that they need a 2second downtime So they can decide: - change the downtime to 2seconds (easy) - change the apps running on the guest to dirty less memory (It dependes on the guest, app, etc). I don't know if anyone is using it at all. Later, Juan.