From: Yury Norov <yury.norov@gmail•com>
To: "Michał Mirosław" <mirq-linux@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: Wed, 1 Dec 2021 16:31:40 -0800 [thread overview]
Message-ID: <20211202003140.GA430494@lapt> (raw)
In-Reply-To: <3CD9ECD8-901E-497B-9AE1-0DDB02346892@rere.qmqm.pl>
On Mon, Nov 29, 2021 at 04:34:07PM +0000, Michał Mirosław wrote:
> 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
Indeed, thanks for pointing to it.
> It would still be more readable, I think.
To be honest, I'm not sure that
bitmap_weight_cmp(..., N-1) <= 0
would be an obvious replacement for the original
bitmap_weight(...) < N
comparing to
bitmap_weight_lt(..., N)
I think the best thing I can do is to add bitmap_weight_cmp() as
you suggested, and turn lt and others to be wrappers on it. This
will let people choose a better function in each case.
I also think that for v2 it would be better to drop the conversion
for short bitmaps, except for switching to bitmap_empty(), because
in that case readability wins over performance; if no objections.
Thanks,
Yury
prev parent reply other threads:[~2021-12-02 6:20 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
2021-11-29 16:34 ` Michał Mirosław
2021-12-02 0:31 ` Yury Norov [this message]
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