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

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Message-ID: <20091202094250.GB31517@wotan.suse.de>
Date:	Wed, 2 Dec 2009 10:42:50 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Liuweni <qingshenlwy@...il.com>, 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>,
	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 07:00:41AM -0700, Matthew Wilcox wrote:
> On Tue, Dec 01, 2009 at 09:21:32PM +0800, Liuweni wrote:
> >  *	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?

Yeah I agree. And then we can debate the usefulness of that workload.

iunique is not used in many filesystems.  If performance really becomes a
problem, then you most likely need another data structure. A per-sb ida or
something would come to mind, but I expect any filesystem that cares already
has a decent inode number allocation so iunique is just a simple hack for
those that don't care so much.

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