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

[Nick Piggin <npiggin@suse.de>]

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.