linux-kernel - Re: [PATCH v1 1/1] cpumask: Don't waste memory for sysfs cpulist nodes

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Date:   Thu, 22 Sep 2022 22:46:48 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v1 1/1] cpumask: Don't waste memory for sysfs cpulist
 nodes

On Thu, Sep 22, 2022 at 10:34:47PM +0300, Andy Shevchenko wrote:
> Currently the approximation is used which wastes the more memory
> the more CPUs are present on the system. Proposed change calculates
> the exact maximum needed in the worst case:
> 
>   NR_CPUS	old		new
>   -------	---		---
>   1 .. 1860	4096		4096
>   ...		...		...
>   2*4096	28672		19925
>   4*4096	57344		43597
>   8*4096	114688		92749
>   16*4096	229376		191053
>   32*4096	458752		403197
>   64*4096	917504		861949
>   128*4096	1835008		1779453
>   256*4096	3670016		3670016
> 
> Under the hood the reccurent formula is being used:
>   (5 - 0) * 2 +
>     (50 - 5) * 3 +
>       (500 - 50) * 4 +
>         (5000 - 500) * 5 +
>           ...
>             (X[i] - X[i-1]) * i
> 
> which allows to count the exact maximum length in the worst case,
> i.e. when each second CPU is being listed. For less than 1861 and
> more than 1 million CPUs the old is being used.

Scratch this because for the range 1171..1860 the new is better,
I'll update commit message and comment in v2 sending soon.

-- 
With Best Regards,
Andy Shevchenko