From: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
To: Yury Norov <yury.norov@gmail.com>
Cc: linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH 0/9] lib/bitmap: optimize bitmap_weight() usage
Date: Mon, 29 Nov 2021 16:34:07 +0000 [thread overview]
Message-ID: <3CD9ECD8-901E-497B-9AE1-0DDB02346892@rere.qmqm.pl> (raw)
In-Reply-To: <20211129063839.GA338729@lapt>
Dnia 29 listopada 2021 06:38:39 UTC, Yury Norov <yury.norov@gmail.com> napisał/a:
>On Sun, Nov 28, 2021 at 07:03:41PM +0100, mirq-test@rere.qmqm.pl wrote:
>> On Sat, Nov 27, 2021 at 07:56:55PM -0800, Yury Norov wrote:
>> > In many cases people use bitmap_weight()-based functions like this:
>> >
>> > if (num_present_cpus() > 1)
>> > do_something();
>> >
>> > This may take considerable amount of time on many-cpus machines because
>> > num_present_cpus() will traverse every word of underlying cpumask
>> > unconditionally.
>> >
>> > We can significantly improve on it for many real cases if stop traversing
>> > the mask as soon as we count present cpus to any number greater than 1:
>> >
>> > if (num_present_cpus_gt(1))
>> > do_something();
>> >
>> > To implement this idea, the series adds bitmap_weight_{eq,gt,le}
>> > functions together with corresponding wrappers in cpumask and nodemask.
>>
>> Having slept on it I have more structured thoughts:
>>
>> First, I like substituting bitmap_empty/full where possible - I think
>> the change stands on its own, so could be split and sent as is.
>
>Ok, I can do it.
>
>> I don't like the proposed API very much. One problem is that it hides
>> the comparison operator and makes call sites less readable:
>>
>> bitmap_weight(...) > N
>>
>> becomes:
>>
>> bitmap_weight_gt(..., N)
>>
>> and:
>> bitmap_weight(...) <= N
>>
>> becomes:
>>
>> bitmap_weight_lt(..., N+1)
>> or:
>> !bitmap_weight_gt(..., N)
>>
>> I'd rather see something resembling memcmp() API that's known enough
>> to be easier to grasp. For above examples:
>>
>> bitmap_weight_cmp(..., N) > 0
>> bitmap_weight_cmp(..., N) <= 0
>> ...
>
>bitmap_weight_cmp() cannot be efficient. Consider this example:
>
>bitmap_weight_lt(1000 0000 0000 0000, 1) == false
> ^
> stop here
>
>bitmap_weight_cmp(1000 0000 0000 0000, 1) == 0
> ^
> stop here
>
>I agree that '_gt' is less verbose than '>', but the advantage of
>'_gt' over '>' is proportional to length of bitmap, and it means
>that this API should exist.
Thank you for the example. Indeed, for less-than to be efficient here you would need to replace
bitmap_weight_cmp(..., N) < 0
with
bitmap_weight_cmp(..., N-1) <= 0
It would still be more readable, I think.
Best Regards
Michał Mirosław
next prev parent reply other threads:[~2021-11-29 16:36 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-28 3:56 [PATCH 0/9] lib/bitmap: optimize bitmap_weight() usage Yury Norov
2021-11-28 3:56 ` [PATCH 1/9] lib/bitmap: add bitmap_weight_{eq,gt,le} Yury Norov
2021-11-28 3:56 ` [PATCH 2/9] lib/bitmap: implement bitmap_{empty,full} with bitmap_weight_eq() Yury Norov
2021-11-28 4:37 ` Michał Mirosław
2021-11-28 6:27 ` Yury Norov
2021-11-28 18:10 ` Michał Mirosław
2021-12-14 19:43 ` Yury Norov
2021-12-15 8:40 ` David Laight
2021-12-15 17:45 ` Yury Norov
2021-11-28 3:56 ` [PATCH 3/9] all: replace bitmap_weigth() with bitmap_{empty,full,eq,gt,le} Yury Norov
2021-11-28 4:47 ` Michał Mirosław
2021-11-28 8:01 ` Greg Kroah-Hartman
2021-11-28 3:56 ` [PATCH 4/9] tools: sync bitmap_weight() usage with the kernel Yury Norov
2021-11-28 3:57 ` [PATCH 5/9] lib/cpumask: add cpumask_weight_{eq,gt,le} Yury Norov
2021-11-28 3:57 ` [PATCH 6/9] lib/nodemask: add nodemask_weight_{eq,gt,le} Yury Norov
2021-11-28 3:57 ` [PATCH 7/9] lib/cpumask: add num_{possible,present,active}_cpus_{eq,gt,le} Yury Norov
2021-11-28 4:56 ` Michał Mirosław
2021-11-28 5:09 ` Michał Mirosław
2021-11-28 6:34 ` Yury Norov
2021-11-28 17:07 ` Joe Perches
2021-11-28 17:43 ` Yury Norov
2021-11-28 17:54 ` Dennis Zhou
2021-11-28 18:47 ` Yury Norov
2021-11-28 17:56 ` Emil Renner Berthing
2021-11-28 17:57 ` Joe Perches
2021-11-28 3:57 ` [PATCH 8/9] lib/nodemask: add num_node_state_eq() Yury Norov
2021-11-28 3:57 ` [PATCH 9/9] MAINTAINERS: add cpumask and nodemask files to BITMAP_API Yury Norov
2021-11-28 11:08 ` [PATCH 0/9] lib/bitmap: optimize bitmap_weight() usage Nicholas Piggin
2021-11-28 23:36 ` Yury Norov
2021-11-28 18:03 ` mirq-test
2021-11-28 18:05 ` Michał Mirosław
2021-11-29 6:38 ` Yury Norov
2021-11-29 16:34 ` Michał Mirosław [this message]
2021-12-02 0:31 ` Yury Norov
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