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

[David Edmondson]

On Wednesday, 2021-03-03 at 19:47:20 +08, Kunkun Jiang wrote:

> 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.

No, I explained myself poorly.

> 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. 😅

I don't think that your version was wrong, because it returned early
from the loop if there were no candidate pages.

>>    }
>>    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.

dme.
-- 
Too much information, running through my brain.