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Date:   Thu, 13 Feb 2020 18:48:06 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Wei Yang <richardw.yang@linux.intel.com>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, shakeelb@google.com,
        yang.shi@linux.alibaba.com
Subject: Re: [RFC Patch] mm/vmscan.c: not inherit classzone_idx from previous
 reclaim
Message-ID: <20200214024806.GU7778@bombadil.infradead.org>
References: <20200209074145.31389-1-richardw.yang@linux.intel.com>
 <20200211104223.GL3466@techsingularity.net>
 <20200212022554.GA7855@richard>
 <20200212074333.GM3466@techsingularity.net>
 <20200214020515.GC20833@richard>
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On Fri, Feb 14, 2020 at 10:05:15AM +0800, Wei Yang wrote:
> On Wed, Feb 12, 2020 at 07:43:33AM +0000, Mel Gorman wrote:
> >Broadly speaking it was driven by cases whereby kswapd either a) fell
> >asleep prematurely and there were many stalls in direct reclaim before
> >kswapd recovered, b) stalls in direct reclaim immediately after kswapd went
> >to sleep or c) kswapd reclaimed for lower zones and went to sleep while
> >parallel tasks were direct reclaiming in higher zones or higher orders.
> 
> Thanks for your explanation. I am trying to understand the connection between
> those cases and the behavior of kswapd.
> 
> In summary, all three cases are related to direct reclaim, while happens in
> three different timing of kswapd:

Reclaim performed by kswapd is the opposite of direct reclaim.  Direct
reclaim is reclaim initiated by a task which is trying to allocate memory.
If a task cannot perform direct reclaim itself, it may ask kswapd to
attempt to reclaim memory for it.