diff mbox series

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

Message ID 20210814211713.180533-12-yury.norov@gmail.com (mailing list archive)
State New, archived
Headers show
Series Resend bitmap patches | expand

Commit Message

Yury Norov Aug. 14, 2021, 9:17 p.m. UTC 
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().

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
---
 include/linux/find.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Petr Mladek Aug. 26, 2021, 1:57 p.m. UTC | #1 
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.


> --- a/include/linux/find.h
> +++ b/include/linux/find.h
> @@ -280,7 +280,7 @@ unsigned long find_next_bit_le(const void *addr, unsigned
>  #endif
>  
>  #define for_each_set_bit(bit, addr, size) \
> -	for ((bit) = find_first_bit((addr), (size));		\
> +	for ((bit) = find_next_bit((addr), (size), 0);		\
>  	     (bit) < (size);					\
>  	     (bit) = find_next_bit((addr), (size), (bit) + 1))
>  

It is not a big deal. I just think that the original code is slightly
more self-explaining.

Best Regards,
Petr
Yury Norov Aug. 26, 2021, 9:09 p.m. UTC | #2 
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

Thanks,
Yury
 
> > --- a/include/linux/find.h
> > +++ b/include/linux/find.h
> > @@ -280,7 +280,7 @@ unsigned long find_next_bit_le(const void *addr, unsigned
> >  #endif
> >  
> >  #define for_each_set_bit(bit, addr, size) \
> > -	for ((bit) = find_first_bit((addr), (size));		\
> > +	for ((bit) = find_next_bit((addr), (size), 0);		\
> >  	     (bit) < (size);					\
> >  	     (bit) = find_next_bit((addr), (size), (bit) + 1))
> >  
> 
> It is not a big deal. I just think that the original code is slightly
> more self-explaining.
> 
> Best Regards,
> Petr
Petr Mladek Aug. 30, 2021, 12:12 p.m. UTC | #3 
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
Yury Norov Aug. 30, 2021, 4:15 p.m. UTC | #4 
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
diff mbox series

Patch

diff --git a/include/linux/find.h b/include/linux/find.h
index 4500e8ab93e2..ae9ed52b52b8 100644
--- a/include/linux/find.h
+++ b/include/linux/find.h
@@ -280,7 +280,7 @@  unsigned long find_next_bit_le(const void *addr, unsigned
 #endif
 
 #define for_each_set_bit(bit, addr, size) \
-	for ((bit) = find_first_bit((addr), (size));		\
+	for ((bit) = find_next_bit((addr), (size), 0);		\
 	     (bit) < (size);					\
 	     (bit) = find_next_bit((addr), (size), (bit) + 1))
 
@@ -291,7 +291,7 @@  unsigned long find_next_bit_le(const void *addr, unsigned
 	     (bit) = find_next_bit((addr), (size), (bit) + 1))
 
 #define for_each_clear_bit(bit, addr, size) \
-	for ((bit) = find_first_zero_bit((addr), (size));	\
+	for ((bit) = find_next_zero_bit((addr), (size), 0);	\
 	     (bit) < (size);					\
 	     (bit) = find_next_zero_bit((addr), (size), (bit) + 1))