Re: [PATCH 2/2] lru: allow large batched add large folio to lru list

[Yin Fengwei <fengwei.yin@intel.com>]

On 4/17/23 20:25, Matthew Wilcox wrote:
> On Mon, Apr 17, 2023 at 03:56:43PM +0800, Yin Fengwei wrote:
>> Currently, large folio is not batched added to lru list. Which
>> cause high lru lock contention after enable large folio for
>> anonymous mappping.
> 
> Obviously, I think we should be doing a batched add, but I don't think
> this is right.
> 
>> @@ -54,7 +57,12 @@ static inline unsigned pagevec_space(struct pagevec *pvec)
>>  static inline unsigned pagevec_add(struct pagevec *pvec, struct page *page)
>>  {
>>  	pvec->pages[pvec->nr++] = page;
>> -	return pagevec_space(pvec);
>> +	pvec->pages_nr += compound_nr(page);
>> +
>> +	if (pvec->pages_nr > PAGEVEC_SIZE)
>> +		return 0;
>> +	else
>> +		return pagevec_space(pvec);
> 
> I assume your thinking here is that we should limit the number of pages
> in the batches, but I think we should allow the number of folios to reach
> PAGEVEC_SIZE before we drain the batch onto the LRU list.  That will
> reduce the contention on the LRU lock even further.

Yes. The first thought in my mind was to limit the number of folios also.

But the concern is that large folio has wider range of size. In the extreme
case, if all batched large folios has 2M size, with PAGEVEC_SIZE as 15,
one batch could have 30M memory. Which could be too large for some usages.


Regards
Yin, Fengwei

>