linux-kernel - Re: [patch 2/2] mm: vmscan: drop nr_force_scan[] from get_scan

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Date:	Fri, 12 Aug 2011 16:04:08 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>, Ying Han <yinghan@...gle.com>,
	Balbir Singh <bsingharora@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [patch 2/2] mm: vmscan: drop nr_force_scan[] from get_scan_count

On Fri, Aug 12, 2011 at 3:58 PM, Johannes Weiner <jweiner@...hat.com> wrote:
> On Fri, Aug 12, 2011 at 08:44:34AM +0900, Minchan Kim wrote:
>> On Fri, Aug 12, 2011 at 5:31 AM, Johannes Weiner <jweiner@...hat.com> wrote:
>> > The nr_force_scan[] tuple holds the effective scan numbers for anon
>> > and file pages in case the situation called for a forced scan and the
>> > regularly calculated scan numbers turned out zero.
>> >
>> > However, the effective scan number can always be assumed to be
>> > SWAP_CLUSTER_MAX right before the division into anon and file.  The
>> > numerators and denominator are properly set up for all cases, be it
>> > force scan for just file, just anon, or both, to do the right thing.
>> >
>> > Signed-off-by: Johannes Weiner <jweiner@...hat.com>
>>
>> Reviewed-by: Minchan Kim <minchan.kim@...il.com>
>
> Thanks.
>
>> There is a nitpick at below.
>
>> > @@ -1927,20 +1917,10 @@ out:
>> >                scan = zone_nr_lru_pages(zone, sc, l);
>> >                if (priority || noswap) {
>> >                        scan >>= priority;
>> > +                       if (!scan && force_scan)
>> > +                               scan = SWAP_CLUSTER_MAX;
>> >                        scan = div64_u64(scan * fraction[file], denominator);
>> >                }
>> > -
>> > -               /*
>> > -                * If zone is small or memcg is small, nr[l] can be 0.
>> > -                * This results no-scan on this priority and priority drop down.
>> > -                * For global direct reclaim, it can visit next zone and tend
>> > -                * not to have problems. For global kswapd, it's for zone
>> > -                * balancing and it need to scan a small amounts. When using
>> > -                * memcg, priority drop can cause big latency. So, it's better
>> > -                * to scan small amount. See may_noscan above.
>> > -                */
>>
>> Please move this comment with tidy-up at where making force_scan true.
>> Of course, we can find it by git log[246e87a9393] but as I looked the
>> git log, it explain this comment indirectly and it's not
>> understandable to newbies. I think this comment is more understandable
>> than changelog in git.
>
> I guess you are right, I am a bit overeager when deleting comments.
> How is this?
>
> ---
> From: Johannes Weiner <jweiner@...hat.com>
> Subject: [patch] mm: vmscan: drop nr_force_scan[] from get_scan_count
>
> The nr_force_scan[] tuple holds the effective scan numbers for anon
> and file pages in case the situation called for a forced scan and the
> regularly calculated scan numbers turned out zero.
>
> However, the effective scan number can always be assumed to be
> SWAP_CLUSTER_MAX right before the division into anon and file.  The
> numerators and denominator are properly set up for all cases, be it
> force scan for just file, just anon, or both, to do the right thing.
>
> Signed-off-by: Johannes Weiner <jweiner@...hat.com>
Reviewed-by: Minchan Kim <minchan.kim@...il.com>

Thanks, Hannes.
-- 
Kind regards,
Minchan Kim
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