Re: [PATCH 11/17] find: micro-optimize for_each_{set,clear}_bit()

Petr Mladek
2021-08-30T12:12:59Z

On Thu 2021-08-26 14:09:55, Yury Norov wrote:
> On Thu, Aug 26, 2021 at 03:57:13PM +0200, Petr Mladek wrote:
> > On Sat 2021-08-14 14:17:07, Yury Norov wrote:
> > > The macros iterate thru all set/clear bits in a bitmap. They search a
> > > first bit using find_first_bit(), and the rest bits using find_next_bit().
> > >
> > > Since find_next_bit() is called shortly after find_first_bit(), we can
> > > save few lines of I-cache by not using find_first_bit().
> >
> > Is this only a speculation or does it fix a real performance problem?
> >
> > The macro is used like:
> >
> > for_each_set_bit(bit, addr, size) {
> > fn(bit);
> > }
> >
> > IMHO, the micro-opimization does not help when fn() is non-trivial.
>
> The effect is measurable:
>
> Start testing for_each_bit()
> for_each_set_bit: 15296 ns, 1000 iterations
> for_each_set_bit_from: 15225 ns, 1000 iterations
>
> Start testing for_each_bit() with cash flushing
> for_each_set_bit: 547626 ns, 1000 iterations
> for_each_set_bit_from: 497899 ns, 1000 iterations
>
> Refer this:
>
> https://www.mail-archive.com/dri-devel@lists.freedesktop.org/msg356151.html

I see. The results look convincing on the first look.

But I am still not sure. This patch is basically contradicting many
other patches from this patchset:

+ 5th patch optimizes find_first_and_bit() and proves that it is
much faster:

Before (#define find_first_and_bit(...) find_next_and_bit(..., 0):
Start testing find_bit() with random-filled bitmap
[ 140.291468] find_first_and_bit: 46890919 ns, 32671 iterations
Start testing find_bit() with sparse bitmap
[ 140.295028] find_first_and_bit: 7103 ns, 1 iterations

After:
Start testing find_bit() with random-filled bitmap
[ 162.574907] find_first_and_bit: 25045813 ns, 32846 iterations
Start testing find_bit() with sparse bitmap
[ 162.578458] find_first_and_bit: 4900 ns, 1 iterations

=> saves 46% in random bitmap
saves 31% in sparse bitmap

+ 6th, 7th, and 9th patch makes the code use find_first_bit()
because it is faster than find_next_bit(mask, size, 0);

+ Now, 11th (this) patch replaces find_first_bit() with
find_next_bit(mask, size, 0) because find_first_bit()
makes things slower. It is suspicious at minimum.

By other words. The I-cache could safe 10% in one case.
But find_first_bit() might safe 46% in random case.

Does I-cache cost more than the faster code?

Or was for_each_set_bit() tested only with a bitmap
where find_first_bit() optimization did not help much?

How would for_each_set_bit() work with random bitmap?
How does it work with larger bitmaps?

Best Regards,
Petr