Re: [PATCH v3 3/3] migration/ram: Optimize ram_save_host_page()

[Kunkun Jiang]

Hi,

On 2021/3/5 22:30, Peter Xu wrote:
> On Fri, Mar 05, 2021 at 03:50:35PM +0800, 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().
> Is there any measurement done?
I tested it on Kunpeng 920.  VM params: 1U 4G( page size 1G).
The time of ram_save_host_page() in the last round of ram saving:
before optimize: 9250us               after optimize: 34us
> This looks like an optimization, but to me it seems to have changed a lot
> context that it doesn't need to... Do you think it'll also work to just look up
> dirty again and update pss->page properly if migration_bitmap_clear_dirty()
> returned zero?
>
> Thanks,
This just inverted the body of the loop, suggested by @David Edmondson.
Here is the v2[1]. Do you mean to change it like this?

[1]: 
20210301082132.1107-4-jiangkunkun@huawei.com/">http://patchwork.ozlabs.org/project/qemu-devel/patch/20210301082132.1107-4-jiangkunkun@huawei.com/

Thanks,
Kunkun Jiang
> > Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> > Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
> > ---
> >   migration/ram.c | 39 +++++++++++++++++++--------------------
> >   1 file changed, 19 insertions(+), 20 deletions(-)
> >
> > diff --git a/migration/ram.c b/migration/ram.c
> > index 9fc5b2997c..28215aefe4 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -1991,6 +1991,8 @@ static int ram_save_host_page(RAMState *rs, 
> > PageSearchStatus *pss,
> >       int 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;
> >   
> > @@ -2003,30 +2005,27 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
> >           int pages_this_iteration = 0;
> >   
> >           /* Check if the page is dirty and send it if it is */
> > -        if (!migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
> > -            pss->page++;
> > -            continue;
> > -        }
> > -
> > -        pages_this_iteration = ram_save_target_page(rs, pss, last_stage);
> > -        if (pages_this_iteration < 0) {
> > -            return pages_this_iteration;
> > -        }
> > +        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;
> > -        pss->page++;
> > -        /*
> > -         * Allow rate limiting to happen in the middle of huge pages if
> > -         * something is sent in the current iteration.
> > -         */
> > -        if (pagesize_bits > 1 && pages_this_iteration > 0) {
> > -            migration_rate_limit();
> > +            pages += pages_this_iteration;
> > +            /*
> > +             * Allow rate limiting to happen in the middle of huge pages if
> > +             * something is sent in the current iteration.
> > +             */
> > +            if (pagesize_bits > 1 && pages_this_iteration > 0) {
> > +                migration_rate_limit();
> > +            }
> >           }
> > -    } while ((pss->page & (pagesize_bits - 1)) &&
> > +        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 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