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

[Kunkun Jiang <jiangkunkun@huawei.com>]

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.