Re: [Qemu-devel] [PATCH v7 25/42] Postcopy: Maintain sentmap and calculate discard

[Dr. David Alan Gilbert]

* Juan Quintela (address@hidden) wrote:
> "Dr. David Alan Gilbert (git)" <address@hidden> wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> >
> > Where postcopy is preceeded by a period of precopy, the destination will
> > have received pages that may have been dirtied on the source after the
> > page was sent.  The destination must throw these pages away before
> > starting it's CPUs.
> >
> > Maintain a 'sentmap' of pages that have already been sent.
> > Calculate list of sent & dirty pages
> > Provide helpers on the destination side to discard these.
> >
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> 
> Not a patch without a suggestion O:-)
> 
> > ---
> >  include/migration/migration.h    |  12 +++
> >  include/migration/postcopy-ram.h |  35 +++++++
> >  include/qemu/typedefs.h          |   1 +
> >  migration/migration.c            |   1 +
> >  migration/postcopy-ram.c         | 108 +++++++++++++++++++++
> >  migration/ram.c                  | 203 
> > ++++++++++++++++++++++++++++++++++++++-
> >  migration/savevm.c               |   2 -
> >  trace-events                     |   5 +
> >  8 files changed, 363 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/migration/migration.h b/include/migration/migration.h
> > index 2a22381..4c6cf95 100644
> > --- a/include/migration/migration.h
> > +++ b/include/migration/migration.h
> > @@ -114,6 +114,13 @@ struct MigrationState
> >  
> >      /* Flag set once the migration has been asked to enter postcopy */
> >      bool start_postcopy;
> > +
> > +    /* bitmap of pages that have been sent at least once
> > +     * only maintained and used in postcopy at the moment
> > +     * where it's used to send the dirtymap at the start
> > +     * of the postcopy phase
> > +     */
> > +    unsigned long *sentmap;
> >  };
> 
> We can use this sentmap for zero page optimization.  If page is on
> sentmap, we need to sent a zero page, otherwise, just sent sentmap at
> the end of migration and clean everything not there?

Just as a compact way of sending zero pages? I'm not sure it would help.

> > +/*
> > + * Discard the contents of memory start..end inclusive.
> > + * We can assume that if we've been called postcopy_ram_hosttest returned 
> > true
> > + */
> > +int postcopy_ram_discard_range(MigrationIncomingState *mis, uint8_t *start,
> > +                               uint8_t *end)
> > +{
> > +    trace_postcopy_ram_discard_range(start, end);
> > +    if (madvise(start, (end-start)+1, MADV_DONTNEED)) {
> 
> Can we s/end/lenght/ and adjust everywhere?

Done - partially; everything that works in bytes is now start & length,
everything that works in indexes into the RAM bitmap is still start/end,
since generally that's what they're working with already.

> Not here, but putting a comment explaining where magic 12 cames from on
> definition of constant?

Done.

> I think that the sentbitmap bits could we used without the rest.

Possibly, I can kind of see it's useful - but I'm not convinced what else for
yet.

Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK