linux-kernel - Re: [PATCH -mmotm] mm: init_per_zone_pages_min - get rid of sqrt call on small machines

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Message-ID: <alpine.DEB.2.00.0905052334391.9824@chino.kir.corp.google.com>
Date:	Tue, 5 May 2009 23:36:38 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LMMML <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mmotm] mm: init_per_zone_pages_min - get rid of sqrt
 call on small machines

On Wed, 6 May 2009, Cyrill Gorcunov wrote:

> Index: linux-2.6.git/mm/page_alloc.c
> =====================================================================
> --- linux-2.6.git.orig/mm/page_alloc.c
> +++ linux-2.6.git/mm/page_alloc.c
> @@ -4610,11 +4610,15 @@ static int __init init_per_zone_pages_mi
>  
>  	lowmem_kbytes = nr_free_buffer_pages() * (PAGE_SIZE >> 10);
>  
> -	min_free_kbytes = int_sqrt(lowmem_kbytes * 16);
> -	if (min_free_kbytes < 128)
> +	/* for small values we may eliminate sqrt operation completely */
> +	if (lowmem_kbytes < 1024)
>  		min_free_kbytes = 128;
> -	if (min_free_kbytes > 65536)
> -		min_free_kbytes = 65536;
> +	else {
> +		min_free_kbytes = int_sqrt(lowmem_kbytes * 16);
> +		if (min_free_kbytes > 65536)
> +			min_free_kbytes = 65536;
> +	}
> +
>  	setup_per_zone_pages_min();
>  	setup_per_zone_lowmem_reserve();
>  	setup_per_zone_inactive_ratio();

For a function that's called once, this just isn't worth it.  int_sqrt() 
isn't expensive enough to warrant the assault on the readability of the 
code.
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