From: Yury Norov <yury.norov@gmail•com>
To: mirq-test@rere•qmqm.pl
Cc: Juri Lelli <juri.lelli@redhat•com>, Andrew Lunn <andrew@lunn•ch>,
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Subject: Re: [PATCH 0/9] lib/bitmap: optimize bitmap_weight() usage
Date: Sun, 28 Nov 2021 22:38:39 -0800 [thread overview]
Message-ID: <20211129063839.GA338729@lapt> (raw)
In-Reply-To: <YaPEfZ0t9UFGwpml@qmqm.qmqm.pl>
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.
> This would also make the implementation easier in not having to
> copy and paste the code three times. Could also use a simple
> optimization reducing code size:
In the next version I'll reduce code duplication like this:
bool bitmap_eq(..., N);
bool bitmap_ge(..., N);
#define bitmap_weight_gt(..., N) bitmap_weight_ge(..., N + 1)
#define bitmap_weight_lt(..., N) !bitmap_weight_ge(..., N)
#define bitmap_weight_le(..., N) !bitmap_weight_gt(..., N)
Thanks,
Yury
next prev parent reply other threads:[~2021-11-29 7:39 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 ` [PATCH 2/9] lib/bitmap: implement bitmap_{empty,full} " Michał Mirosław
2021-11-28 6:27 ` Yury Norov
2021-11-28 18:10 ` Michał Mirosław
2021-12-14 19:43 ` [PATCH 2/9] lib/bitmap: implement bitmap_{empty, full} " Yury Norov
2021-12-15 8:40 ` [PATCH 2/9] lib/bitmap: implement bitmap_{empty,full} " David Laight
2021-12-15 17:45 ` [PATCH 2/9] lib/bitmap: implement bitmap_{empty, full} " 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 ` [PATCH 3/9] all: replace bitmap_weigth() with bitmap_{empty,full,eq,gt,le} 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 ` [PATCH 7/9] lib/cpumask: add num_{possible,present,active}_cpus_{eq,gt,le} 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 ` [PATCH 7/9] lib/cpumask: add num_{possible, present, active}_cpus_{eq, gt, le} Emil Renner Berthing
2021-11-28 17:57 ` [PATCH 7/9] lib/cpumask: add num_{possible,present,active}_cpus_{eq,gt,le} 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 18:05 ` Michał Mirosław
2021-11-28 18:03 ` mirq-test
2021-11-29 6:38 ` Yury Norov [this message]
2021-11-29 16:34 ` Michał Mirosław
2021-12-02 0:31 ` Yury Norov
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