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Message-ID: <CAAH8bW8piLSCYLKjVXYV45cJeHApFX3Z3G=Zx-nap3yCA1=DDg@mail.gmail.com>
Date: Fri, 11 Dec 2020 09:20:23 -0800
From: Yury Norov <yury.norov@...il.com>
To: Levi Yun <ppbuk5246@...il.com>
Cc: dushistov@...l.ru, Arnd Bergmann <arnd@...db.de>,
Andrew Morton <akpm@...ux-foundation.org>,
"Gustavo A. R. Silva" <gustavo@...eddedor.com>,
William Breathitt Gray <vilhelm.gray@...il.com>,
richard.weiyang@...ux.alibaba.com,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
joseph.qi@...ux.alibaba.com, skalluru@...vell.com,
Josh Poimboeuf <jpoimboe@...hat.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/find_bit_bench: fix the unmatched iterations cnt
On Fri, Dec 11, 2020 at 12:50 AM Levi Yun <ppbuk5246@...il.com> wrote:
>
> We should have same iteration count when we walk the same bitmap
> regardless of using find_next_bit or find_last_b
I think it's not that important, because the difference is not measurable.
But if this part raises questions, I have nothing against aligning numbers.
> When we run the find_bit_benchmark.ko, we sometime get
> unmatched iterations count below:
>
> Start testing find_bit() with random-filled bitmap
> [+...] find_next_bit: 875085 ns, 163755 iterations <
> [+...] find_next_zero_bit: 865319 ns, 163926 iterations
> [+...] find_last_bit: 611807 ns, 163756 iterations <
> [+...] find_first_bit: 1601016 ns, 16335 iterations
> [+...] find_next_and_bit: 400645 ns, 74040 iterations
> [+...]
> Start testing find_bit() with sparse bitmap
> [+...] find_next_bit: 9942 ns, 654 iterations
> [+...] find_next_zero_bit: 1678445 ns, 327027 iterations
> [+...] find_last_bit: 7131 ns, 654 iterations
> [+...] find_first_bit: 551383 ns, 654 iterations
> [+...] find_next_and_bit: 3027 ns, 1 iterations
>
> Normally, this is happen when the last bit of bitmap was set.
Can you please confirm that for bitmap 0001,
test_find_{first,next,next_and}_bit reports cnt == 0, and
test_find_last_bit() reports 1?
> This patch fix the unmatched iterations count between
> test_find_next_bit and test_find_last_bit.
>
> Signed-off-by: Levi Yun <ppbuk5246@...il.com>
> ---
> lib/find_bit_benchmark.c | 30 ++++++++++++++++--------------
> 1 file changed, 16 insertions(+), 14 deletions(-)
>
> diff --git a/lib/find_bit_benchmark.c b/lib/find_bit_benchmark.c
> index 5637c5711db9..766e0487852b 100644
> --- a/lib/find_bit_benchmark.c
> +++ b/lib/find_bit_benchmark.c
> @@ -35,14 +35,14 @@ static DECLARE_BITMAP(bitmap2, BITMAP_LEN) __initdata;
> */
> static int __init test_find_first_bit(void *bitmap, unsigned long len)
> {
> - unsigned long i, cnt;
> + unsigned long i = 0, cnt = 0;
> ktime_t time;
>
> time = ktime_get();
> - for (cnt = i = 0; i < len; cnt++) {
> + do {
> i = find_first_bit(bitmap, len);
> __clear_bit(i, bitmap);
> - }
> + } while (i++ < len && ++cnt);
What for this check against ++cnt? I doubt that the counter can overflow.
> time = ktime_get() - time;
> pr_err("find_first_bit: %18llu ns, %6ld\n", time, cnt);
>
> @@ -51,12 +51,13 @@ static int __init test_find_first_bit(void *bitmap, unsigned long len)
>
> static int __init test_find_next_bit(const void *bitmap, unsigned long len)
> {
> - unsigned long i, cnt;
> + unsigned long i = 0, cnt = 0;
> ktime_t time;
>
> time = ktime_get();
> - for (cnt = i = 0; i < BITMAP_LEN; cnt++)
> - i = find_next_bit(bitmap, BITMAP_LEN, i) + 1;
> + do {
> + i = find_next_bit(bitmap, BITMAP_LEN, i);
> + } while (i++ < BITMAP_LEN && ++cnt);
> time = ktime_get() - time;
> pr_err("find_next_bit: %18llu ns, %6ld iterations\n", time, cnt);
>
> @@ -65,12 +66,13 @@ static int __init test_find_next_bit(const void *bitmap, unsigned long len)
>
> static int __init test_find_next_zero_bit(const void *bitmap, unsigned long len)
> {
> - unsigned long i, cnt;
> + unsigned long i = 0, cnt = 0;
> ktime_t time;
>
> time = ktime_get();
> - for (cnt = i = 0; i < BITMAP_LEN; cnt++)
> - i = find_next_zero_bit(bitmap, len, i) + 1;
> + do {
> + i = find_next_zero_bit(bitmap, len, i);
> + } while (i++ < BITMAP_LEN && ++cnt);
> time = ktime_get() - time;
> pr_err("find_next_zero_bit: %18llu ns, %6ld iterations\n", time, cnt);
>
> @@ -84,12 +86,11 @@ static int __init test_find_last_bit(const void *bitmap, unsigned long len)
>
> time = ktime_get();
> do {
> - cnt++;
> l = find_last_bit(bitmap, len);
> if (l >= len)
> break;
> len = l;
> - } while (len);
> + } while (len >= 0 && ++cnt);
Why this?
> time = ktime_get() - time;
> pr_err("find_last_bit: %18llu ns, %6ld iterations\n", time, cnt);
>
> @@ -99,12 +100,13 @@ static int __init test_find_last_bit(const void *bitmap, unsigned long len)
> static int __init test_find_next_and_bit(const void *bitmap,
> const void *bitmap2, unsigned long len)
> {
> - unsigned long i, cnt;
> + unsigned long i = 0, cnt = 0;
> ktime_t time;
>
> time = ktime_get();
> - for (cnt = i = 0; i < BITMAP_LEN; cnt++)
> - i = find_next_and_bit(bitmap, bitmap2, BITMAP_LEN, i + 1);
> + do {
> + i = find_next_and_bit(bitmap, bitmap2, BITMAP_LEN, i);
> + } while (i++ < BITMAP_LEN && ++cnt);
Do you experience the same problem with find_next_and_bit() as well?
> time = ktime_get() - time;
> pr_err("find_next_and_bit: %18llu ns, %6ld iterations\n", time, cnt);
>
> --
> 2.27.0
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