Re: [PATCH] migration: Rate limit inside host pages

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Juan Quintela (quintela@redhat.com) wrote:
> "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >
> > When using hugepages, rate limiting is necessary within each huge
> > page, since a 1G huge page can take a significant time to send, so
> > you end up with bursty behaviour.
> >
> > Fixes: 4c011c37ecb3 ("postcopy: Send whole huge pages")
> > Reported-by: Lin Ma <LMa@suse.com>
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> 
> Reviewed-by: Juan Quintela <quintela@redhat.com>
> 
> I can agree that rate limit needs to be done for huge pages.
> 
> > diff --git a/migration/ram.c b/migration/ram.c
> > index a4ae3b3120..a9177c6a24 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -2616,6 +2616,8 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
> >  
> >          pages += tmppages;
> >          pss->page++;
> > +        /* Allow rate limiting to happen in the middle of huge pages */
> > +        migration_rate_limit();
> >      } while ((pss->page & (pagesize_bits - 1)) &&
> >               offset_in_ramblock(pss->block, pss->page << TARGET_PAGE_BITS));
> >  
> 
> But is doing the rate limit for each page, no?  Even when not using huge
> pages.

Right.

> Not that it should be a big issue (performance wise).
> Have you done any meassuremnet?

I've just given it a quick run; it still seems to be hitting ~9.5Gbps on
my 10Gbps interface; so it doesn't seem to be the limit on that.

Dave

> 
> 
> Later, Juan.
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK