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Re: [Qemu-devel] [PATCH v4 37/47] Page request: Consume pages off the po
From: |
David Gibson |
Subject: |
Re: [Qemu-devel] [PATCH v4 37/47] Page request: Consume pages off the post-copy queue |
Date: |
Tue, 27 Jan 2015 15:38:11 +1100 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Wed, Jan 14, 2015 at 08:13:27PM +0000, Dr. David Alan Gilbert wrote:
> * David Gibson (address@hidden) wrote:
> > On Fri, Oct 03, 2014 at 06:47:43PM +0100, Dr. David Alan Gilbert (git)
> > wrote:
> > > From: "Dr. David Alan Gilbert" <address@hidden>
> > >
> > > 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 <address@hidden>
> > > ---
> > > 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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