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

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Date:   Mon, 6 Mar 2017 14:42:25 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     Andrew Morton <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, Wei.

On Fri, Feb 17, 2017 at 10:12:31PM +0800, Wei Yang wrote:
> > And compare the ruling with the iteration for the loop to be (1UL <<
> > 5) and (1UL << 19).
> > The runtime is 0.00s and 0.04s respectively. The absolute value is not much.

systemd-analyze usually does a pretty good job of breaking down which
phase took how long.  It might be worthwhile to test whether the
improvement is actually visible during the boot.

> >> * Do we really need to add full reverse iterator to just get the
> >>   highest section number?
> >>
> >
> > You are right. After I sent out the mail, I realized just highest pfn
> > is necessary.

That said, getting efficient is always great as long as the added
complexity is justifiably small enough.  If you can make the change
simple enough, it'd be a lot easier to merge.

Thanks.

-- 
tejun