linux-kernel - Re: [PATCH 2/3]fs/inode: iunique() Optimize Performance

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Date:	Wed, 25 Nov 2009 07:17:28 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Liuweni <qingshenlwy@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	viro <viro@...iv.linux.org.uk>, akpm <akpm@...ux-foundation.org>,
	jack <jack@...e.cz>, npiggin <npiggin@...e.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	strongzgy <strongzgy@...il.com>, xgr178 <xgr178@....com>,
	Liu Hui <onlyflyer@...il.com>
Subject: Re: [PATCH 2/3]fs/inode: iunique() Optimize Performance

On Wed, Nov 25, 2009 at 10:12:19PM +0800, Liuweni wrote:
> @@ -605,8 +605,8 @@ static unsigned long hash(struct super_block *sb, unsigned long hashval)
>  {
>  	unsigned long tmp;
>  
> -	tmp = (hashval * (unsigned long)sb) ^ (GOLDEN_RATIO_PRIME + hashval) /
> -			L1_CACHE_BYTES;
> +	tmp = (hashval * (unsigned long)sb) ^ (GOLDEN_RATIO_PRIME + hashval) >>
> +			L1_CACHE_SHIFT;
>  	tmp = tmp ^ ((tmp ^ GOLDEN_RATIO_PRIME) >> I_HASHBITS);
>  	return tmp & I_HASHMASK;
>  }

Have you compared the compiler output before/after your change?  I'd be
amazed if GCC isn't able to optimise division-by-a-constant-power-of-two
into shift-by-constant.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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