From: Yury Norov <yury.norov@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: mm-commits@vger.kernel.org, jserv@ccns.ncku.edu.tw, visitorckw@gmail.com
Subject: Re: + bitops-optimize-fns-for-improved-performance.patch added to mm-nonmm-unstable branch
Date: Fri, 26 Apr 2024 12:48:48 -0700 [thread overview]
Message-ID: <CAAH8bW9o+d48_84mWXwYkVgMzY+__UxfBnuzauci4qgt1jFZ+Q@mail.gmail.com> (raw)
In-Reply-To: <20240426190857.BB28FC2BD10@smtp.kernel.org>
On Fri, Apr 26, 2024 at 12:08 PM Andrew Morton
<akpm@linux-foundation.org> wrote:
>
>
> The patch titled
> Subject: bitops: optimize fns() for improved performance
> has been added to the -mm mm-nonmm-unstable branch. Its filename is
> bitops-optimize-fns-for-improved-performance.patch
>
> This patch will shortly appear at
> https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/bitops-optimize-fns-for-improved-performance.patch
>
> This patch will later appear in the mm-nonmm-unstable branch at
> git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
>
> Before you just go and hit "reply", please:
> a) Consider who else should be cc'ed
> b) Prefer to cc a suitable mailing list as well
> c) Ideally: find the original patch on the mailing list and do a
> reply-to-all to that, adding suitable additional cc's
>
> *** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
>
> The -mm tree is included into linux-next via the mm-everything
> branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
> and is updated there every 2-3 working days
>
> ------------------------------------------------------
> From: Kuan-Wei Chiu <visitorckw@gmail.com>
> Subject: bitops: optimize fns() for improved performance
> Date: Fri, 26 Apr 2024 11:51:52 +0800
>
> 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.
>
> The following microbenchmark data, conducted on my x86-64 machine, shows
> the execution time (in microseconds) required for 1000000 test data
> generated by get_random_u64() and executed by fns() under different values
> of n:
>
> +-----+---------------+---------------+
> | n | time_old | time_new |
> +-----+---------------+---------------+
> | 0 | 29194 | 25878 |
> | 1 | 25510 | 25497 |
> | 2 | 27836 | 25721 |
> | 3 | 30140 | 25673 |
> | 4 | 32569 | 25426 |
> | 5 | 34792 | 25690 |
> | 6 | 37117 | 25651 |
> | 7 | 39742 | 25383 |
> | 8 | 42360 | 25657 |
> | 9 | 44672 | 25897 |
> | 10 | 47237 | 25819 |
> | 11 | 49884 | 26530 |
> | 12 | 51864 | 26647 |
> | 13 | 54265 | 28915 |
> | 14 | 56440 | 28373 |
> | 15 | 58839 | 28616 |
> | 16 | 62383 | 29128 |
> | 17 | 64257 | 30041 |
> | 18 | 66805 | 29773 |
> | 19 | 69368 | 33203 |
> | 20 | 72942 | 33688 |
> | 21 | 77006 | 34518 |
> | 22 | 80926 | 34298 |
> | 23 | 85723 | 35586 |
> | 24 | 90324 | 36376 |
> | 25 | 95992 | 37465 |
> | 26 | 101101 | 37599 |
> | 27 | 106520 | 37466 |
> | 28 | 113287 | 38163 |
> | 29 | 120552 | 38810 |
> | 30 | 128040 | 39373 |
> | 31 | 135624 | 40500 |
> | 32 | 142580 | 40343 |
> | 33 | 148915 | 40460 |
> | 34 | 154005 | 41294 |
> | 35 | 157996 | 41730 |
> | 36 | 160806 | 41523 |
> | 37 | 162975 | 42088 |
> | 38 | 163426 | 41530 |
> | 39 | 164872 | 41789 |
> | 40 | 164477 | 42505 |
> | 41 | 164758 | 41879 |
> | 42 | 164182 | 41415 |
> | 43 | 164842 | 42119 |
> | 44 | 164881 | 42297 |
> | 45 | 164870 | 42145 |
> | 46 | 164673 | 42066 |
> | 47 | 164616 | 42051 |
> | 48 | 165055 | 41902 |
> | 49 | 164847 | 41862 |
> | 50 | 165171 | 41960 |
> | 51 | 164851 | 42089 |
> | 52 | 164763 | 41717 |
> | 53 | 164635 | 42154 |
> | 54 | 164757 | 41983 |
> | 55 | 165095 | 41419 |
> | 56 | 164641 | 42381 |
> | 57 | 164601 | 41654 |
> | 58 | 164864 | 41834 |
> | 59 | 164594 | 41920 |
> | 60 | 165207 | 42020 |
> | 61 | 165056 | 41185 |
> | 62 | 165160 | 41722 |
> | 63 | 164923 | 41702 |
> | 64 | 164777 | 41880 |
> +-----+---------------+---------------+
Hi Kuan-Wei,
I didn't receive the original email for some reason...
We've got a performance test for the function in find_bit_benchmark.
Can you print before/after here?
Thanks,
Yury
> Link: https://lkml.kernel.org/r/20240426035152.956702-1-visitorckw@gmail.com
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
> Cc: Yury Norov <yury.norov@gmail.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
> include/linux/bitops.h | 12 ++++--------
> 1 file changed, 4 insertions(+), 8 deletions(-)
>
> --- a/include/linux/bitops.h~bitops-optimize-fns-for-improved-performance
> +++ a/include/linux/bitops.h
> @@ -254,16 +254,12 @@ static inline unsigned long __ffs64(u64
> */
> static inline unsigned long fns(unsigned long word, unsigned int n)
> {
> - unsigned int bit;
> + unsigned int i;
>
> - while (word) {
> - bit = __ffs(word);
> - if (n-- == 0)
> - return bit;
> - __clear_bit(bit, &word);
> - }
> + for (i = 0; word && i < n; i++)
> + word &= word - 1;
>
> - return BITS_PER_LONG;
> + return word ? __ffs(word) : BITS_PER_LONG;
> }
>
> /**
> _
>
> Patches currently in -mm which might be from visitorckw@gmail.com are
>
> bitops-optimize-fns-for-improved-performance.patch
>
next prev parent reply other threads:[~2024-04-26 19:49 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-26 19:08 + bitops-optimize-fns-for-improved-performance.patch added to mm-nonmm-unstable branch Andrew Morton
2024-04-26 19:48 ` Yury Norov [this message]
2024-04-27 5:33 ` Kuan-Wei Chiu
2024-04-28 16:08 ` Yury Norov
2024-04-29 8:05 ` Rasmus Villemoes
2024-04-29 15:40 ` Kuan-Wei Chiu
2024-04-29 16:34 ` Yury Norov
2024-04-29 17:06 ` Kuan-Wei Chiu
2024-04-29 16:26 ` Yury Norov
2024-04-29 15:31 ` Kuan-Wei Chiu
-- strict thread matches above, loose matches on Subject: below --
2024-05-02 15:18 Andrew Morton
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