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

On Thu 2021-08-26 14:09:55, Yury Norov wrote:

On Thu, Aug 26, 2021 at 03:57:13PM +0200, Petr Mladek wrote:

quoted

On Sat 2021-08-14 14:17:07, Yury Norov wrote:

quoted

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