Re: [PATCH RFC] mm,vmscan: only evict file pages when we have plenty

[Rik van Riel <riel@redhat.com>]

On 10/31/2012 11:39 AM, Johannes Weiner wrote:
> On Tue, Oct 30, 2012 at 02:42:04PM -0400, Rik van Riel wrote:
>> If we have more inactive file pages than active file pages, we
>> skip scanning the active file pages alltogether, with the idea
>> that we do not want to evict the working set when there is
>> plenty of streaming IO in the cache.
>>
>> However, the code forgot to also skip scanning anonymous pages
>> in that situation.  That lead to the curious situation of keeping
>> the active file pages protected from being paged out when there
>> are lots of inactive file pages, while still scanning and evicting
>> anonymous pages.
>>
>> This patch fixes that situation, by only evicting file pages
>> when we have plenty of them and most are inactive.
>>
>> Signed-off-by: Rik van Riel <riel@redhat.com>
>> ---
>>   mm/vmscan.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 2624edc..1a53fbb 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1686,6 +1686,15 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>>   			fraction[1] = 0;
>>   			denominator = 1;
>>   			goto out;
>> +		} else if (!inactive_file_is_low_global(zone)) {
>> +			/*
>> +			 * There is enough inactive page cache, do not
>> +			 * reclaim anything from the working set right now.
>> +			 */
>> +			fraction[0] = 0;
>> +			fraction[1] = 1;
>> +			denominator = 1;
>> +			goto out;
>
> Is there a specific reason for making this exclusive to global
> reclaim?  The "force scan anon when file is low" HAS to be specific to
> global reclaim because swapping may not be allowed in memcg limit
> reclaim, but not scanning anon when there is enough easy page cache is
> a legitimate memcg limit reclaim thing to do as well.

Good point.  I guess this check would work fine inside
cgroup reclaim, too.

Want to give that a try?

>
> I.e. could this check be moved just below the
>
> 	/* If we have no swap space, do not bother scanning anon pages. */
> 	if (!sc->may_swap || (nr_swap_pages <= 0)) {
> 		noswap = 1;
> 		fraction[0] = 0;
> 		fraction[1] = 1;
> 		denominator = 1;
> 		goto out;
> 	}
>
> section?
>

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