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