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From: Michael Ellerman <mpe@ellerman.id.au>
To: Yury Norov <ynorov@caviumnetworks.com>, linux-kernel@vger.kernel.org
Cc: Yury Norov <ynorov@caviumnetworks.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Clement Courbet <courbet@google.com>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>
Subject: Re: [PATCH] lib: test module for find_*_bit() functions
In-Reply-To: <20171109140714.13168-1-ynorov@caviumnetworks.com>
References: <20171109140714.13168-1-ynorov@caviumnetworks.com>
Date: Sun, 12 Nov 2017 22:33:55 +1100
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Yury Norov <ynorov@caviumnetworks.com> writes:

> find_bit functions are widely used in the kernel, including hot paths.
> This module tests performance of that functions in 2 typical scenarios:
> randomly filled bitmap with relatively equal distribution of set and
> cleared bits, and sparse bitmap which has 1 set bit for 500 cleared bits.
>
> On ThunderX machine:
>
> 		 Start testing find_bit() with random-filled bitmap
> [1032111.632383] find_next_bit:		240043 cycles,	164062 iterations
> [1032111.647236] find_next_zero_bit:	312848 cycles,	163619 iterations
> [1032111.661585] find_last_bit:		193748 cycles,	164062 iterations
> [1032113.450517] find_first_bit:	177720874 cycles,	164062 iterations
> [1032113.462930]
> 		 Start testing find_bit() with sparse bitmap
> [1032113.477229] find_next_bit:		3633 cycles,	656 iterations
> [1032113.494281] find_next_zero_bit:	620399 cycles,	327025 iterations
> [1032113.506723] find_last_bit:		3038 cycles,	656 iterations
> [1032113.524485] find_first_bit:	691407 cycles,	656 iterations

Have you thought about timing it rather than using get_cycles()?

get_cycles() has the downside that it can't be compared across different
architectures or even platforms within an architecture.

cheers