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

[Peter Xu <peterx@redhat.com>]

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?

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,

> 
> 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
> 

-- 
Peter Xu