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Date:   Tue, 13 Feb 2018 15:55:11 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     Jonathan Corbet <corbet@lwn.net>, Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@suse.de>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-doc@vger.kernel.org
Subject: Re: [patch 1/2] mm, page_alloc: extend kernelcore and movablecore
 for percent
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On Tue, 13 Feb 2018, Andrew Morton wrote:

> > Both kernelcore= and movablecore= can be used to define the amount of
> > ZONE_NORMAL and ZONE_MOVABLE on a system, respectively.  This requires
> > the system memory capacity to be known when specifying the command line,
> > however.
> > 
> > This introduces the ability to define both kernelcore= and movablecore=
> > as a percentage of total system memory.  This is convenient for systems
> > software that wants to define the amount of ZONE_MOVABLE, for example, as
> > a proportion of a system's memory rather than a hardcoded byte value.
> > 
> > To define the percentage, the final character of the parameter should be
> > a '%'.
> 
> Is this fine-grained enough?  We've had percentage-based tunables in
> the past, and 10 years later when systems are vastly larger, 1% is too
> much.
> 

They still have the (current) ability to define the exact amount of bytes 
down to page sized granularity, whereas 1% would yield 40GB on a 4TB 
system.  I'm not sure that people will want any finer-grained control if 
defining the proportion of the system for kernelcore.  They do have the 
ability with the existing interface, though, if they want to be that 
precise.

(This is a cop out for not implementing some fractional percentage parser, 
 although that would be possible as a more complete solution.)