Re: [PATCH v2 3/3] migration/ram: Optimize ram_save_host_page()

[Kunkun Jiang]

On 2021/3/3 16:56, David Edmondson wrote:
> On Monday, 2021-03-01 at 16:21:32 +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().
> This is cleaner, thank you.
>
> I was hoping to just invert the body of the loop - something like
> (completely untested):
Sorry for my misunderstanding.
I will improve it in the next version.
> do {
>    int pages_this_iteration = 0;
>
>    /* Check if the page is dirty and, if so, send it. */
>    if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
>      pages_this_iteration = ram_save_target_page(rs, pss, last_stage);
>      if (pages_this_iteration < 0) {
>        return pages_this_iteration;
>      }
>
>      pages += pages_this_iteration;
>
>      /*
>       * Allow rate limiting to happen in the middle of huge pages if
>       * the current iteration sent something.
>       */
>      if (pagesize_bits > 1 && pages_this_iteration > 0) {
>        migration_rate_limit();
>      }
I missed the case that the value of pages_this_iteration is 0. 😅
>    }
>    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));
> /* The offset we leave with is the min boundary of host page and block */
> pss->page = MIN(pss->page, hostpage_boundary) - 1;

Best Regards.

Kunkun Jiang

> > Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> > Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
> > ---
> >   migration/ram.c | 12 +++++++-----
> >   1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/migration/ram.c b/migration/ram.c
> > index 3a9115b6dc..a1374db356 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -1991,6 +1991,8 @@ 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;
> >   
> > @@ -2002,7 +2004,7 @@ 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++;
> > +            pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
> >               continue;
> >           }
> >   
> > @@ -2012,16 +2014,16 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
> >           }
> >   
> >           pages += tmppages;
> > -        pss->page++;
> > +        pss->page = migration_bitmap_find_dirty(rs, pss->block, 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)) &&
> > +    } while ((pss->page < hostpage_boundary) &&
> >                offset_in_ramblock(pss->block,
> >                                   ((ram_addr_t)pss->page) << TARGET_PAGE_BITS));
> > -    /* 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.