linux-kernel - Re: [PATCH] lib/find_bit_bench: fix the unmatched iterations cnt

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Date:   Fri, 11 Dec 2020 18:11:54 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Levi Yun <ppbuk5246@...il.com>
Cc:     dushistov@...l.ru, arnd@...db.de, akpm@...ux-foundation.org,
        gustavo@...eddedor.com, vilhelm.gray@...il.com,
        richard.weiyang@...ux.alibaba.com, joseph.qi@...ux.alibaba.com,
        skalluru@...vell.com, yury.norov@...il.com, jpoimboe@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/find_bit_bench: fix the unmatched iterations cnt

On Fri, Dec 11, 2020 at 05:50:39PM +0900, Levi Yun wrote:
> We should have same iteration count when we walk the same bitmap
> regardless of using find_next_bit or find_last_bit.

I didn't understand why is so (I mean "same", I think you rather talking about
same order of amount of itterations).

> 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.
> 
> This patch fix the unmatched iterations count between
> test_find_next_bit and test_find_last_bit.

Can you provide before and after to compare?

-- 
With Best Regards,
Andy Shevchenko