linux-kernel - Re: [RFC PATCH 2/2] mm/sparse: add last_section_nr in sparse

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Message-ID: <20170211022400.GA19050@mtj.duckdns.org>
Date:   Sat, 11 Feb 2017 11:24:00 +0900
From:   Tejun Heo <tj@...nel.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] mm/sparse: add last_section_nr in sparse_init()
 to reduce some iteration cycle

Hello,

On Sat, Feb 11, 2017 at 10:18:29AM +0800, Wei Yang wrote:
> During the sparse_init(), it iterate on each possible section. On x86_64,
> it would always be (2^19) even there is not much memory. For example, on a
> typical 4G machine, it has only (2^5) to (2^6) present sections. This
> benefits more on a system with smaller memory.
> 
> This patch calculates the last section number from the highest pfn and use
> this as the boundary of iteration.

* How much does this actually matter?  Can you measure the impact?

* Do we really need to add full reverse iterator to just get the
  highest section number?

Thanks.

-- 
tejun