linux-kernel - Re: [PATCH 01/10] mm: vmscan: Limit the number of pages kswapd reclaims at each priority

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Message-ID: <514BA118.5000305@gmail.com>
Date:	Fri, 22 Mar 2013 08:08:56 +0800
From:	Will Huck <will.huckk@...il.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Michal Hocko <mhocko@...e.cz>, Mel Gorman <mgorman@...e.de>,
	Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Zlatko Calusic <zcalusic@...sync.net>,
	Johannes Weiner <hannes@...xchg.org>,
	dormando <dormando@...ia.net>,
	Satoru Moriya <satoru.moriya@....com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/10] mm: vmscan: Limit the number of pages kswapd reclaims
 at each priority

Hi Rik,
On 03/21/2013 08:52 AM, Rik van Riel wrote:
> On 03/20/2013 12:18 PM, Michal Hocko wrote:
>> On Sun 17-03-13 13:04:07, Mel Gorman wrote:
>> [...]
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 88c5fed..4835a7a 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -2593,6 +2593,32 @@ static bool prepare_kswapd_sleep(pg_data_t 
>>> *pgdat, int order, long remaining,
>>>   }
>>>
>>>   /*
>>> + * kswapd shrinks the zone by the number of pages required to reach
>>> + * the high watermark.
>>> + */
>>> +static void kswapd_shrink_zone(struct zone *zone,
>>> +                   struct scan_control *sc,
>>> +                   unsigned long lru_pages)
>>> +{
>>> +    unsigned long nr_slab;
>>> +    struct reclaim_state *reclaim_state = current->reclaim_state;
>>> +    struct shrink_control shrink = {
>>> +        .gfp_mask = sc->gfp_mask,
>>> +    };
>>> +
>>> +    /* Reclaim above the high watermark. */
>>> +    sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));
>>
>> OK, so the cap is at high watermark which sounds OK to me, although I
>> would expect balance_gap being considered here. Is it not used
>> intentionally or you just wanted to have a reasonable upper bound?
>>
>> I am not objecting to that it just hit my eyes.
>
> This is the maximum number of pages to reclaim, not the point
> at which to stop reclaiming.

What's the difference between the maximum number of pages to reclaim and 
the point at which to stop reclaiming?

>
> I assume Mel chose this value because it guarantees that enough
> pages will have been freed, while also making sure that the value
> is scaled according to zone size (keeping pressure between zones
> roughly equal).
>

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