Re: [PATCH] fs: use approximate counter values for inodes and dentries. (was Re: [patch] fs: use fast counters for vfs caches)

[Nick Piggin <npiggin@gmail.com>]

On Thu, Dec 9, 2010 at 6:45 PM, Dave Chinner <david@fromorbit.com> wrote:
> On Thu, Dec 09, 2010 at 05:16:44PM +1100, Nick Piggin wrote:
>> On Thu, Dec 09, 2010 at 04:43:43PM +1100, Dave Chinner wrote:
>> > On Mon, Nov 29, 2010 at 09:57:33PM +1100, Nick Piggin wrote:
>> > > Hey,
>> > >
>> > > What was the reason behind not using my approach to use fast per-cpu
>> > > counters for inode and dentry counters, and instead using the
>> > > percpu_counter lib (which is not useful unless very fast approximate
>> > > access to the global counter is required, or performance is not
>> > > critical, which is somewhat of an oxymoron if you're using per-counters
>> > > in the first place). It is a difference between this:
>> >
>> > Hi Nick - sorry for being slow to answer this - I only just found
>> > this email.
>> >
>> > The reason for using the generic counters is because the shrinkers
>> > read the current value of the global counter on every call and hence
>> > they can be read thousands of times a second. The only way to do that
>> > efficiently is to use the approximately value the generic counters
>> > provide.
>>
>> That is not what is happening, though, so I assume that no measurements
>> were done.
>>
>> In fact what happens now is that *both* type of counters use the crappy
>> percpu counter library, and the shrinkers actually do a per-cpu loop
>> over the counters to get the sum.
>
> More likely that the overhead was hidden in the noise on the size of
> machines most people test on.

No. I was referring to the decision to use the heavyweight percpu_counter
code over the superior per cpu data that I was using.

Also, the unrelated change to make nr_unused into per-cpu was not
right, and I will revert that back to a global variable. (again, unless you
have numbers)

> It certainly wasn't measurable on my
> 16p machine, and nobody who reviewed it at the time (ѕeveral people)
> picked it up. So thanks for reviewing it - the simple fix is below.
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
>
> fs: Use approximate values for number of inodes and dentries
>
> From: Dave Chinner <dchinner@redhat.com>

Nack. Can you please address my points and actually explain why this
is better than my proposed approach please?
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