linux-kernel - Re: [PATCH] : A better approach to compute int_sqrt in lib/int

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Date:	Thu, 17 Jul 2008 20:26:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Soumyadip Das Mahapatra <soumya.linux@...oo.com>
Cc:	linux-kernel@...r.kernel.org,
	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>
Subject: Re: [PATCH] : A better approach to compute int_sqrt in
	lib/int_sqrt.c

On Wed, 2008-07-16 at 14:35 -0700, Soumyadip Das Mahapatra wrote:


> 0 It is better because 
>          o it uses only one loop instead of two
>          o contains no division operator (older version has two)
>             which are surely comparatively slow task in computer

As Lennart has said, gcc is smart enough to transform a division by a
power-of-two into shifts.

> 0 Currently find . -name '*.[ch]' | xargs grep int_sqrt gives me this
>         ....
>         ./fs/nfs/write.c:       nfs_congestion_kb = (16*int_sqrt(totalram_pages)) << (PAGE_SHIFT-10);
>         ./drivers/video/fbmon.c:        h_period = int_sqrt(h_period);
>         ./mm/page_alloc.c:      min_free_kbytes = int_sqrt(lowmem_kbytes * 16);
>         ./mm/oom_kill.c:        s = int_sqrt(cpu_time);
>         ./mm/oom_kill.c:        s = int_sqrt(int_sqrt(run_time));
>         ....

fs/nfs/write.c is init code
mm/page_alloc.c is also init code
mm/oom_kill.c isn't a hot path

which leaves the fbmon case, which after a quick peek is setup code, so
not a hot path either.

So while that doesn't preclude us from changing it if you can indeed
show its a better implementation, its not on anybodies hit-list.

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