Re: [Qemu-devel] [PATCH v4 37/47] Page request: Consume pages off the post-copy queue

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Jan 14, 2015 at 08:13:27PM +0000, Dr. David Alan Gilbert wrote:
> * David Gibson (david@gibson.dropbear.id.au) wrote:
> > On Fri, Oct 03, 2014 at 06:47:43PM +0100, Dr. David Alan Gilbert (git) wrote:
> > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > 
> > > When transmitting RAM pages, consume pages that have been queued by
> > > MIG_RPCOMM_REQPAGE commands and send them ahead of normal page scanning.
> > > 
> > > Note:
> > >   a) After a queued page the linear walk carries on from after the
> > > unqueued page; there is a reasonable chance that the destination
> > > was about to ask for other closeby pages anyway.
> > > 
> > >   b) We have to be careful of any assumptions that the page walking
> > > code makes, in particular it does some short cuts on its first linear
> > > walk that break as soon as we do a queued page.
> > > 
> > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > ---
> > >  arch_init.c | 149 ++++++++++++++++++++++++++++++++++++++++++++++++++----------
> > >  1 file changed, 125 insertions(+), 24 deletions(-)
> > > 
> > > diff --git a/arch_init.c b/arch_init.c
> > > index 72f9e17..a945990 100644
> > > +
> > > +        /*
> > > +         * Don't break host-page chunks up with queue items
> > 
> > Why does this matter?
> 
> See the comment you make in a few patches time, it's about being able
> to place the pages atomically on the destination.

Hmm.  But if the destination has to wait for all the pieces of a host
page to arrive anyway, does it really make any difference if they're
contiguous in the stream?

> > > +         * so only unqueue if,
> > > +         *   a) The last item came from the queue anyway
> > > +         *   b) The last sent item was the last target-page in a host page
> > > +         */
> > > +        if (last_was_from_queue || (!last_sent_block) ||
> > > +            ((last_offset & (hps - 1)) == (hps - TARGET_PAGE_SIZE))) {
> > > +            tmpblock = ram_save_unqueue_page(ms, &tmpoffset, &bitoffset);
> > >          }
> > > -        if (offset >= block->length) {
> > > -            offset = 0;
> > > -            block = QTAILQ_NEXT(block, next);
> > > -            if (!block) {
> > > -                block = QTAILQ_FIRST(&ram_list.blocks);
> > > -                complete_round = true;
> > > -                ram_bulk_stage = false;
> > > +
> > > +        if (tmpblock) {
> > > +            /* We've got a block from the postcopy queue */
> > > +            DPRINTF("%s: Got postcopy item '%s' offset=%zx bitoffset=%zx",
> > > +                    __func__, tmpblock->idstr, tmpoffset, bitoffset);
> > > +            /* We're sending this page, and since it's postcopy nothing else
> > > +             * will dirty it, and we must make sure it doesn't get sent again.
> > > +             */
> > > +            if (!migration_bitmap_clear_dirty(bitoffset << TARGET_PAGE_BITS)) {
> > 
> > Ugh.. that's kind of subtle.  I think it would be clearer if you work
> > in terms of a ram_addr_t throughout, rather than "bitoffset" whose
> > meaning is not terribly obvious.
> 
> I've changed it to ram_addr_t as requested; it's slightly clearer but there
> are a few places where we're dealing with the sentmap where we now need to shift
> the other way.  In the end ram_addr_t is really a scaled offset into those
> bitmaps.

Right, but to someone who isn't deeply familiar with the code, they're
more likely to understand what the ram address means than the bitmap
offset.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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