Re: [PATCH 3/3] migration/ram: Optimize ram_save_host_page()

[Kunkun Jiang]

On 2021/2/25 20:48, David Edmondson wrote:
> On Tuesday, 2021-02-23 at 10:16:45 +08, Kunkun Jiang wrote:
>
> > Starting from pss->page, ram_save_host_page() will check every page
> > and send the dirty pages up to the end of the current host page or
> > the boundary of used_length of the block. If the host page size is
> > a huge page, the step "check" will take a lot of time.
> >
> > This will improve performance to use migration_bitmap_find_dirty().
> >
> > Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> > Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
> > ---
> >   migration/ram.c | 14 ++++++++------
> >   1 file changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/migration/ram.c b/migration/ram.c
> > index c7e18dc2fc..c7a2350198 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -1994,6 +1994,8 @@ static int ram_save_host_page(RAMState *rs, 
> > PageSearchStatus *pss,
> >       int tmppages, pages = 0;
> >       size_t pagesize_bits =
> >           qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
> > +    unsigned long hostpage_boundary =
> > +        QEMU_ALIGN_UP(pss->page + 1, pagesize_bits);
> >       unsigned long start_page = pss->page;
> >       int res;
> >   
> > @@ -2005,8 +2007,7 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
> >       do {
> >           /* Check the pages is dirty and if it is send it */
> >           if (!migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
> > -            pss->page++;
> > -            continue;
> > +            goto find_next;
> >           }
> >   
> >           tmppages = ram_save_target_page(rs, pss, last_stage);
> > @@ -2015,16 +2016,17 @@ 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 */
> >           if (pagesize_bits > 1) {
> >               migration_rate_limit();
> >           }
> > -    } while ((pss->page & (pagesize_bits - 1)) &&
> > +find_next:
> > +        pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
> > +    } while ((pss->page < hostpage_boundary) &&
> >                offset_in_ramblock(pss->block,
> >                                   ((ram_addr_t)pss->page) << TARGET_PAGE_BITS));
> This ends up looking very messy, with a goto inside the loop.
>
> Wouldn't it be cleaner to invert the sense of the
> migration_bitmap_clear_dirty() test, such that
> migration_bitmap_find_dirty() is called after the body of the test?
Sorry for the late reply.
Thanks for your advice.  I will post a v2 as soon as possible.

Best Regards.

Kunkun Jiang

> > -    /* The offset we leave with is the last one we looked at */
> > -    pss->page--;
> > +    /* The offset we leave with is the min boundary of host page and block */
> > +    pss->page = MIN(pss->page, hostpage_boundary) - 1;
> >   
> >       res = ram_save_release_protection(rs, pss, start_page);
> >       return (res < 0 ? res : pages);
> > --
> > 2.23.0
> dme.