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Date:   Thu, 4 Apr 2019 12:39:14 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Zhaoyang Huang <huangzhaoyang@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        David Rientjes <rientjes@google.com>,
        Zhaoyang Huang <zhaoyang.huang@unisoc.com>,
        Roman Gushchin <guro@fb.com>, Jeff Layton <jlayton@redhat.com>,
        Matthew Wilcox <mawilcox@microsoft.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm:workingset use real time to judge activity of the
 file page
Message-ID: <20190404163914.GA4229@cmpxchg.org>
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On Thu, Apr 04, 2019 at 11:30:17AM +0800, Zhaoyang Huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> 
> In previous implementation, the number of refault pages is used
> for judging the refault period of each page, which is not precised as
> eviction of other files will be affect a lot on current cache.
> We introduce the timestamp into the workingset's entry and refault ratio
> to measure the file page's activity. It helps to decrease the affection
> of other files(average refault ratio can reflect the view of whole system
> 's memory).

I don't understand what exactly you're saying here, can you please
elaborate?

The reason it's using distances instead of absolute time is because
the ordering of the LRU is relative and not based on absolute time.

E.g. if a page is accessed every 500ms, it depends on all other pages
to determine whether this page is at the head or the tail of the LRU.

So when you refault, in order to determine the relative position of
the refaulted page in the LRU, you have to compare it to how fast that
LRU is moving. The absolute refault time, or the average time between
refaults, is not comparable to what's already in memory.