Re: [Qemu-devel] [PATCH] migration: not send zero page header in ram bulk stage

[Hailiang Zhang]

Hi,

On 2016/1/15 18:24, Li, Liang Z wrote:
> > It seems that this patch is incorrect, if the no-zero pages are zeroed again
> > during !ram_bulk_stage, we didn't send the new zeroed page, there will be
> > an error.
>
> If not in ram_bulk_stage, still send the header, could you explain why it's 
> wrong?
>
> Liang

I have made a mistake, and yes, this patch can speed up the live migration time,
especially when there are many zero pages, it will be more obvious.
I like this idea. Did you test it with postcopy ? Does it break postcopy ?

Thanks,
zhanghailiang

> > > For guest just uses a small portions of RAM, this change can avoid
> > > allocating all the guest's RAM pages in the destination node after
> > > live migration. Another benefit is destination QEMU can save lots of
> > > CPU cycles for zero page checking.
> > >
> > > Signed-off-by: Liang Li <address@hidden>
> > > ---
> > >    migration/ram.c | 10 ++++++----
> > >    1 file changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/migration/ram.c b/migration/ram.c index 4e606ab..c4821d1
> > > 100644
> > > --- a/migration/ram.c
> > > +++ b/migration/ram.c
> > > @@ -705,10 +705,12 @@ static int save_zero_page(QEMUFile *f,
> > RAMBlock
> > > *block, ram_addr_t offset,
> > >
> > >        if (is_zero_range(p, TARGET_PAGE_SIZE)) {
> > >            acct_info.dup_pages++;
> > > -        *bytes_transferred += save_page_header(f, block,
> > > -                                               offset | 
> > > RAM_SAVE_FLAG_COMPRESS);
> > > -        qemu_put_byte(f, 0);
> > > -        *bytes_transferred += 1;
> > > +        if (!ram_bulk_stage) {
> > > +            *bytes_transferred += save_page_header(f, block, offset |
> > > +                                                   RAM_SAVE_FLAG_COMPRESS);
> > > +            qemu_put_byte(f, 0);
> > > +            *bytes_transferred += 1;
> > > +        }
> > >            pages = 1;
> > >        }
>
>
> .