Re: [PATCH] mm, page_alloc: skip zone who has no managed_pages in calculate_totalreserve_pages()

[Wei Yang <richard.weiyang@gmail.com>]

On Mon, Nov 12, 2018 at 09:09:26AM +0100, Michal Hocko wrote:
>On Mon 12-11-18 15:14:04, Wei Yang wrote:
>> Zone with no managed_pages doesn't contribute totalreserv_pages. And the
>> more nodes we have, the more empty zones there are.
>> 
>> This patch skip the zones to save some cycles.
>
>What is the motivation for the patch? Does it really cause any
>measurable difference in performance?
>

The motivation here is to reduce some unnecessary work.

Based on my understanding, almost every node has empty zones, since
zones within a node are ordered in monotonic increasing memory address.

The worst case is all zones has managed_pages. For example, there is
only one node, or configured to have only ZONE_NORMAL and
ZONE_MOVABLE. Otherwise, the more node/zone we have, the more empty
zones there are.

I didn't have detail tests on this patch, since I don't have machine
with large numa nodes. While compared with the following ten lines of
code, this check to skip them is worthwhile to me.


>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> ---
>>  mm/page_alloc.c | 3 +++
>>  1 file changed, 3 insertions(+)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index a919ba5cb3c8..567de15e1106 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -7246,6 +7246,9 @@ static void calculate_totalreserve_pages(void)
>>  			struct zone *zone = pgdat->node_zones + i;
>>  			long max = 0;
>>  
>> +			if (!managed_zone(zone))
>> +				continue;
>> +
>>  			/* Find valid and maximum lowmem_reserve in the zone */
>>  			for (j = i; j < MAX_NR_ZONES; j++) {
>>  				if (zone->lowmem_reserve[j] > max)
>> -- 
>> 2.15.1
>> 
>
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me