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

Yury Norov
2021-08-30T16:15:09Z

On Mon, Aug 30, 2021 at 02:12:49PM +0200, Petr Mladek wrote:
> 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.

Those are different cases. find_first_bit() is approximately twice
faster than find_next_bit, and much smaller. The conclusion is simple:
use 'first' version whenever possible if there's no other considerations.

In case of for_each_bit() macros, however, we have such a consideration.
In contrast to regular pattern, where user calls either first, or next
versions N times, here we call find_first_bit once, and then find_next_bit
N-1 times.

Because we know for sure that we'll call find_next_bit shortly, we can
benefit from locality under heavy pressure on I-cache, if replace 'first'
with 'next'. Consider it as a prefetch mechanism for the following calls
to find_next_bit().

> Does I-cache cost more than the faster code?

In this case cache miss is more expensive.

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

I tried to ensure that the effect of I-cache is real and in this case
more important than code performance, so in the test I called 'first'
once and 'next' twice.

> How would for_each_set_bit() work with random bitmap?

It would work for all bitmaps.

> How does it work with larger bitmaps?

Percentage gain (but not absolute) will decrease proportionally to the
number of calls of find_next_bit() for big N.

Thanks,
Yury

> Best Regards,
> Petr