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SubjectRe: 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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