Re: [Qemu-devel] [PATCH RFC] migration: make sure to run iterate precopy during the bulk stage

[Quan Xu]

on 2018/9/4 17:12, Juan Quintela wrote:
> Quan Xu <address@hidden> wrote:
> >  From 8dbf7370e7ea1caab0b769d0d4dcdd072d14d421 Mon Sep 17 00:00:00 2001
> > From: Quan Xu <address@hidden>
> > Date: Wed, 29 Aug 2018 21:33:14 +0800
> > Subject: [PATCH RFC] migration: make sure to run iterate precopy during the
> >   bulk stage
> >
> > Since the bulk stage assumes in (migration_bitmap_find_dirty) that every
> > page is dirty, return a rough total ram as pending size to make sure that
> > migration thread continues to run iterate precopy during the bulk stage.
> >
> > Otherwise the downtime grows unpredictably, as migration thread needs to
> > send both the rest of pages and dirty pages during complete precopy.
> >
> > Signed-off-by: Quan Xu <address@hidden>
> > ---
> >   migration/ram.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/migration/ram.c b/migration/ram.c
> > index 79c8942..cfa304c 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -3308,7 +3308,8 @@ static void ram_save_pending(QEMUFile *f, void
> > *opaque, uint64_t max_size,
> >           /* We can do postcopy, and all the data is postcopiable */
> >           *res_compatible += remaining_size;
> >       } else {
> > -        *res_precopy_only += remaining_size;
> > +        *res_precopy_only += (rs->ram_bulk_stage ?
> > +                              ram_bytes_total() : remaining_size);
> >       }
> >   }
>
> Hi
>
> I don't oppose the change.
> But what I don't understand is _why_ it is needed (or to say it
> otherwise, how it worked until now). 

I run migration in a slow network throughput (about ~500mbps).
​ in my opion,  as the slow network throughput, there is more 'break' 
during iterate precopy (as the MAX_WAIT).
​as said in patch description, even to send both the rest pages and 
dirty pages, if in a higher network throughput,
​the downtime would look still within an acceptable range.



>  I was wondering about the opposit
> direction, and just initialize the number of dirty pages at the
> beggining of the loop and then let decrease it for each processed page.

I understand your concern. I also wanted to fix as your suggestion. 
however, to me, this would be an overhead to ​​maintain another count 
during migration.


Quan


> I don't remember either how big was the speedud of not walking the
> bitmap on the 1st stage to start with.
>
> Later, Juan.