Re: [patch][rfc] fs: shrink struct dentry

[John Levon <levon@movementarian.org>]

On Tue, Dec 02, 2008 at 08:06:08AM +0100, Nick Piggin wrote:

> > Don't you even have a differential profile showing the impact of
> > removing d_cookie? This hash table lookup will now happen on *every*
> > userspace sample that's processed. That's, uh, a lot.
> 
> I don't know what you mean by every sample that's processed, but
> won't the hash lookup only happen for the *first* time that a given
> name is asked for a dcookie (ie. fast_get_dcookie, which, as I said,
> should actually be moved to fs/dcookies.c)

I mis-read your changes.

> > (By all means make your change, but I don't get how it's OK to regress
> > other code, and provide no evidence at all as to its impact.)
> 
> Tradeoffs are made all the time. This is obviously a good one, and
                                           ^^^^^^^^^^^^^^^^^^^^

By all means make your change, but I don't get how it's OK to regress
other code, and provide no evidence at all as to its impact.

> I provided evidence of the impact of the improvement in the common
> case. I also acknowledge it can slow down the uncommon case, but
> showed ways that can easily be improved. Do you want me to just try
> to make an artificial case where I mmap thousands of tiny shared
> libraries and try to overflow the hash and try to detect a difference?

You haven't even bothered to show that it hasn't affected normal
oprofile use yet.

I can't believe I'm having to argue that you need to test your code. So
I think I'll stop.

> Did you add d_cookie? If so, then surely at the time you must have

It was added along with the rest of oprofile, so I don't have breakout
numbers. I did have oprofile overhead numbers, though I doubt I could
find them now.

john

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