linux-kernel - Re: [PATCH] vmscan: evict use-once pages first (v3)

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Date:	Fri, 01 May 2009 19:05:21 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	kosaki.motohiro@...fujitsu.com, peterz@...radead.org,
	elladan@...imo.com, linux-kernel@...r.kernel.org, tytso@....edu,
	linux-mm@...ck.org
Subject: Re: [PATCH] vmscan: evict use-once pages first (v3)

Andrew Morton wrote:
> On Wed, 29 Apr 2009 13:14:36 -0400
> Rik van Riel <riel@...hat.com> wrote:
> 
>> When the file LRU lists are dominated by streaming IO pages,
>> evict those pages first, before considering evicting other
>> pages.
>>
>> This should be safe from deadlocks or performance problems
>> because only three things can happen to an inactive file page:
>> 1) referenced twice and promoted to the active list
>> 2) evicted by the pageout code
>> 3) under IO, after which it will get evicted or promoted
>>
>> The pages freed in this way can either be reused for streaming
>> IO, or allocated for something else. If the pages are used for
>> streaming IO, this pageout pattern continues. Otherwise, we will
>> fall back to the normal pageout pattern.
>>
>> ..
>>
>> +int mem_cgroup_inactive_file_is_low(struct mem_cgroup *memcg)
>> +{
>> +	unsigned long active;
>> +	unsigned long inactive;
>> +
>> +	inactive = mem_cgroup_get_local_zonestat(memcg, LRU_INACTIVE_FILE);
>> +	active = mem_cgroup_get_local_zonestat(memcg, LRU_ACTIVE_FILE);
>> +
>> +	return (active > inactive);
>> +}
> 
> This function could trivially be made significantly more efficient by
> changing it to do a single pass over all the zones of all the nodes,
> rather than two passes.

How would I do that in a clean way?

The function mem_cgroup_inactive_anon_is_low and
the global versions all do the same.  It would be
nice to make all four of them go fast :)

If there is no standardized infrastructure for
getting multiple statistics yet, I can probably
whip something up.

>>  static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
>>  	struct zone *zone, struct scan_control *sc, int priority)
>>  {
>>  	int file = is_file_lru(lru);
>>  
>> -	if (lru == LRU_ACTIVE_FILE) {
>> +	if (lru == LRU_ACTIVE_FILE && inactive_file_is_low(zone, sc)) {
>>  		shrink_active_list(nr_to_scan, zone, sc, priority, file);
>>  		return 0;
>>  	}
> 
> And it does get called rather often.

Same as inactive_anon_is_low.

Optimizing them might make sense if it turns out to
use a significant amount of CPU.

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