Re: [PATCH v10 03/35] dcache: convert dentry_stat.nr_unused to per-cpu counters

Dave Chinner
2013-06-06T04:02:29Z

On Wed, Jun 05, 2013 at 07:48:01PM -0700, Andrew Morton wrote:
> On Thu, 6 Jun 2013 11:45:09 +1000 Dave Chinner <david@fromorbit.com> wrote:
>
> > Andrew, if you want to push the changes back to generic per-cpu
> > counters through to Linus, then I'll write the patches for you. But
> > - and this is a big but - I'll only do this if you are going to deal
> > with the "performance trumps all other concerns" fanatics over
> > whether it should be merged or not. I have better things to do
> > with my time have a flamewar over trivial details like this.
>
> Please view my comments as a critique of the changelog, not of the code.
>
> There are presumably good (but undisclosed) reasons for going this way,
> but this question is so bleeding obvious that the decision should have
> been addressed up-front and in good detail.

The answer is so bleeding obvious I didn't think it needed to be
documented. ;) i.e. implement it the same way that it's sibling is
implemented because consistency is good....

> And, preferably, with benchmark numbers. Because it might have been
> the wrong decision - stranger things have happened.

I've never been able to measure the difference in fast-path
performance that can be attributed to the generic CPU counters
having more overhead than the special ones. If you've got any
workload where the fast-path counter overhead shows up in a
profile, I'd be very interested....

Cheers,

dave.
--
Dave Chinner
david@fromorbit.com

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