linux-kernel - Re: Re: [PATCH 1/3]fs/inode: iunique() Optimize Performance

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Date:	Tue, 1 Dec 2009 07:00:41 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Liuweni <qingshenlwy@...il.com>
Cc:	strongzgy <strongzgy@...il.com>, xgr178 <xgr178@....com>,
	Liu Hui <onlyflyer@...il.com>, 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>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH 1/3]fs/inode: iunique() Optimize Performance

On Tue, Dec 01, 2009 at 09:21:32PM +0800, Liuweni wrote:
> Hi Matthew Wilcox:
> 	I got the code note as the following. if there is no performance problem, 
> maybe the code note need update.
> 
> 
> ------
>  *	BUGS:
>  *	With a large number of inodes live on the file system this function
>  *	currently becomes quite slow.
> 

I don't believe that comment is correct.  In any case, your optimisation
wouldn't make a lick of difference to the speed; it's a single comparison
in a loop which also calculates a hash, makes a function call, and walks
the length of a hash chain.

The old adage about debugging code, not comments applies here.  Don't take
somebody else's word for it that there's a performance problem here.
Devise a test to demonstrate the performance problem.  Otherwise, how
will you know if you solved it?

-- 
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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