Hello,
This patch series optimizes the fns() function by avoiding repeated
calls to __ffs(). Additionally, tests for fns() have been added in
lib/test_bitops.c.
Changes in v3:
- Move the benchmark test for fns() to lib/test_bitops.c.
- Exclude the overhead of random number generation from the benchmark
result.
- Change the output to print only a total gross instead of each n in
the benchmark result.
- Update the commit message in the second patch.
Changes in v2:
- Add benchmark test for fns() in lib/find_bit_benchmark.c.
- Change the loop in fns() by counting down from n to 0.
- Add find_bit benchmark result for find_nth_bit in commit message.
Link to v2: https://lkml.kernel.org/[email protected]
Link to v1: https://lkml.kernel.org/[email protected]
Kuan-Wei Chiu (2):
lib/test_bitops: Add benchmark test for fns()
bitops: Optimize fns() for improved performance
include/linux/bitops.h | 12 +++---------
lib/test_bitops.c | 22 ++++++++++++++++++++++
2 files changed, 25 insertions(+), 9 deletions(-)
--
2.34.1
Introduce a benchmark test for the fns(). It measures the total time
taken by fns() to process 1,000,000 test data generated using
get_random_long() for each n in the range [0, BITS_PER_LONG).
example:
test_bitops: fns: 5876762553 ns, 64000000 iterations
Signed-off-by: Kuan-Wei Chiu <[email protected]>
---
Changes in v3:
- Move the benchmark test for fns() to lib/test_bitops.c.
- Exclude the overhead of random number generation from the benchmark
result.
- Change the output to print only a total gross instead of each n in
the benchmark result.
lib/test_bitops.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/lib/test_bitops.c b/lib/test_bitops.c
index 3b7bcbee84db..ed939f124417 100644
--- a/lib/test_bitops.c
+++ b/lib/test_bitops.c
@@ -50,6 +50,26 @@ static unsigned long order_comb_long[][2] = {
};
#endif
+static unsigned long buf[1000000];
+
+static int __init test_fns(void)
+{
+ unsigned int i, n;
+ ktime_t time;
+
+ get_random_bytes(buf, sizeof(buf));
+ time = ktime_get();
+
+ for (n = 0; n < BITS_PER_LONG; n++)
+ for (i = 0; i < 1000000; i++)
+ fns(buf[i], n);
+
+ time = ktime_get() - time;
+ pr_err("fns: %18llu ns, %6d iterations\n", time, BITS_PER_LONG * 1000000);
+
+ return 0;
+}
+
static int __init test_bitops_startup(void)
{
int i, bit_set;
@@ -94,6 +114,8 @@ static int __init test_bitops_startup(void)
if (bit_set != BITOPS_LAST)
pr_err("ERROR: FOUND SET BIT %d\n", bit_set);
+ test_fns();
+
pr_info("Completed bitops test\n");
return 0;
--
2.34.1
The current fns() repeatedly uses __ffs() to find the index of the
least significant bit and then clears the corresponding bit using
__clear_bit(). The method for clearing the least significant bit can be
optimized by using word &= word - 1 instead.
Typically, the execution time of one __ffs() plus one __clear_bit() is
longer than that of a bitwise AND operation and a subtraction. To
improve performance, the loop for clearing the least significant bit
has been replaced with word &= word - 1, followed by a single __ffs()
operation to obtain the answer. This change reduces the number of
__ffs() iterations from n to just one, enhancing overall performance.
This modification significantly accelerates the fns() function in the
test_bitops benchmark, improving its speed by approximately 439 times.
Additionally, it enhances the performance of find_nth_bit() in the
find_bit benchmark by approximately 26%.
Before:
test_bitops: fns: 5876762553 ns, 64000000 iterations
find_nth_bit: 4254313 ns, 16525 iterations
After:
test_bitops: fns: 13388431 ns, 64000000 iterations
find_nth_bit: 3362863 ns, 16501 iterations
Signed-off-by: Kuan-Wei Chiu <[email protected]>
---
Changes in v3:
- Add the fns() benchmark result from lib/test_bitops.c to the commit
message.
- Modify the commit message to display only the total gross instead of
each n values in the benchmark result.
include/linux/bitops.h | 12 +++---------
1 file changed, 3 insertions(+), 9 deletions(-)
diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 2ba557e067fe..57ecef354f47 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -254,16 +254,10 @@ static inline unsigned long __ffs64(u64 word)
*/
static inline unsigned long fns(unsigned long word, unsigned int n)
{
- unsigned int bit;
+ while (word && n--)
+ word &= word - 1;
- while (word) {
- bit = __ffs(word);
- if (n-- == 0)
- return bit;
- __clear_bit(bit, &word);
- }
-
- return BITS_PER_LONG;
+ return word ? __ffs(word) : BITS_PER_LONG;
}
/**
--
2.34.1
From: Kuan-Wei Chiu
> Sent: 01 May 2024 08:17
>
> Introduce a benchmark test for the fns(). It measures the total time
> taken by fns() to process 1,000,000 test data generated using
> get_random_long() for each n in the range [0, BITS_PER_LONG).
>
> example:
> test_bitops: fns: 5876762553 ns, 64000000 iterations
Great benchmark....
The compiler almost certainly optimises it all away.
Assigning the result of fns() to a file scope (global) volatile int
should stop that happening.
And a real test would actually check the result - just in case
someone does something silly.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
Hi David,
On Sun, May 05, 2024 at 01:03:23PM +0000, David Laight wrote:
> From: Kuan-Wei Chiu
> > Sent: 01 May 2024 08:17
> >
> > Introduce a benchmark test for the fns(). It measures the total time
> > taken by fns() to process 1,000,000 test data generated using
> > get_random_long() for each n in the range [0, BITS_PER_LONG).
> >
> > example:
> > test_bitops: fns: 5876762553 ns, 64000000 iterations
>
> Great benchmark....
>
> The compiler almost certainly optimises it all away.
>
> Assigning the result of fns() to a file scope (global) volatile int
> should stop that happening.
>
Thank you for your review. There is an updated v5 of this patch [1],
which has already been accepted and included in Yury's bitmap-for-next
branch of the bitmap tree. In the v5 patch, we have addressed the issue
you mentioned regarding the use of volatile variables to avoid compiler
optimizations.
> And a real test would actually check the result - just in case
> someone does something silly.
>
The fns() function is mainly a helper for find_nth_bit(), so its
accuracy should have been checked in find_nth_bit()'s tests. If you
want unit tests for fns() here too, that sounds good to me, but it
would likely be a separate patch. I'm happy to do it if you'd like.
Regards,
Kuan-Wei
> David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>
On Mon, May 06, 2024 at 01:27:25AM +0800, Kuan-Wei Chiu wrote:
> Hi David,
>
> On Sun, May 05, 2024 at 01:03:23PM +0000, David Laight wrote:
> > From: Kuan-Wei Chiu
> > > Sent: 01 May 2024 08:17
> > >
> > > Introduce a benchmark test for the fns(). It measures the total time
> > > taken by fns() to process 1,000,000 test data generated using
> > > get_random_long() for each n in the range [0, BITS_PER_LONG).
> > >
> > > example:
> > > test_bitops: fns: 5876762553 ns, 64000000 iterations
> >
> > Great benchmark....
> >
> > The compiler almost certainly optimises it all away.
> >
> > Assigning the result of fns() to a file scope (global) volatile int
> > should stop that happening.
> >
> Thank you for your review. There is an updated v5 of this patch [1],
> which has already been accepted and included in Yury's bitmap-for-next
> branch of the bitmap tree. In the v5 patch, we have addressed the issue
> you mentioned regarding the use of volatile variables to avoid compiler
> optimizations.
>
[1]: https://lore.kernel.org/lkml/[email protected]/
> > And a real test would actually check the result - just in case
> > someone does something silly.
> >
> The fns() function is mainly a helper for find_nth_bit(), so its
> accuracy should have been checked in find_nth_bit()'s tests. If you
> want unit tests for fns() here too, that sounds good to me, but it
> would likely be a separate patch. I'm happy to do it if you'd like.
>
> Regards,
> Kuan-Wei
>
> > David
> >
> > -
> > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> > Registration No: 1397386 (Wales)
> >