From: Nikita Danilov <nikita@clusterfs.com>
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Message-ID: <17299.1331.368159.374754@gargle.gargle.HOWL>
Date: Sun, 4 Dec 2005 18:03:15 +0300
To: Wu Fengguang <wfg@mail.ustc.edu.cn>
Cc: Andrew Morton <akpm@osdl.org>,
       Linux Kernel Mailing List <Linux-Kernel@vger.kernel.org>
Subject: Re: [PATCH 01/16] mm: delayed page activation
In-Reply-To: <20051204134818.GA4305@mail.ustc.edu.cn>
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Wu Fengguang writes:
 > On Sun, Dec 04, 2005 at 03:11:28PM +0300, Nikita Danilov wrote:
 > > Wu Fengguang writes:
 > >  > When a page is referenced the second time in inactive_list, mark it with
 > >  > PG_activate instead of moving it into active_list immediately. The actual
 > >  > moving work is delayed to vmscan time.
 > >  > 
 > >  > This implies two essential changes:
 > >  > - keeps the adjecency of pages in lru;
 > > 
 > > But this change destroys LRU ordering: at the time when shrink_list()
 > > inspects PG_activate bit, information about order in which
 > > mark_page_accessed() was called against pages is lost. E.g., suppose
 > 
 > Thanks.
 > But this order of re-access time may be pointless. In fact the original
 > mark_page_accessed() is doing another inversion: inversion of page lifetime.
 > In the word of CLOCK-Pro, a page first being re-accessed has lower

The brave new world of CLOCK-Pro is still yet to happen, right?

 > inter-reference distance, and therefore should be better protected(if ignore
 > possible read-ahead effects). If we move re-accessed pages immediately into
 > active_list, we are pushing them closer to danger of eviction.

Huh? Pages in the active list are closer to the eviction? If it is
really so, then CLOCK-pro hijacks the meaning of active list in a very
unintuitive way. In the current MM active list is supposed to contain
hot pages that will be evicted last.

Anyway, these issues should be addressed in CLOCK-pro
implementation. Current MM tries hard to maintain LRU approximation in
both active and inactive lists.

[...]

 > 
 > Though I have a strong feeling that with the extra PG_activate bit, the
 > +                       ClearPageReferenced(page);
 > line should be removed. That is, let the extra reference record live through it.
 > The point is to smooth out the inter-reference distance. Imagine the following
 > situation:
 > 
 > -      +            -   +           +   -                   -   +              -
 > 1                   2                   3                   4                  5
 >         +: reference time
 >         -: shrink_list time
 > 
 > One page have an average inter-reference distance that is smaller than the
 > inter-scan distance. But the distances vary a bit. Here we'd better let the
 > reference count accumulate, or at the 3rd shrink_list time it will be evicted.

I think this is pretty normal and acceptable variance. Idea is that when
system is short on memory scan rate increases together with the
precision of reference tracking.

[...]

 > 
 > Thanks,

Che-che,

 > Wu

Nikita.
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