linux-kernel - Re: [RANDOM] Move two variables to read

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Message-ID: <20071216130028.GB14233@stusta.de>
Date:	Sun, 16 Dec 2007 14:00:28 +0100
From:	Adrian Bunk <bunk@...nel.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Matt Mackall <mpm@...enic.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RANDOM] Move two variables to read_mostly section to save
	memory

On Sun, Dec 16, 2007 at 12:45:01PM +0100, Eric Dumazet wrote:
> While examining vmlinux namelist on i686, I noticed :
>
> c0581300 D random_table
> c0581480 d input_pool
> c0581580 d random_read_wakeup_thresh
> c0581584 d random_write_wakeup_thresh
> c0581600 d blocking_pool
>
> That means that the two integers random_read_wakeup_thresh and 
> random_write_wakeup_thresh use a full cache entry (128 bytes).
>
> Moving them to read_mostly section can shrinks vmlinux by 120 bytes.
>
> # size vmlinux*
>    text    data     bss     dec     hex filename
> 4835553  450210  610304 5896067  59f783 vmlinux.after_patch
> 4835553  450330  610304 5896187  59f7fb vmlinux.before_patch
>
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 5fee056..af48e86 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -256,14 +256,14 @@
>   * The minimum number of bits of entropy before we wake up a read on
>   * /dev/random.  Should be enough to do a significant reseed.
>   */
> -static int random_read_wakeup_thresh = 64;
> +static int random_read_wakeup_thresh __read_mostly = 64;
>  
>  /*
>   * If the entropy count falls under this number of bits, then we
>   * should wake up processes which are selecting or polling on write
>   * access to /dev/random.
>   */
> -static int random_write_wakeup_thresh = 128;
> +static int random_write_wakeup_thresh __read_mostly = 128;

Please never ever do such ugly and unmaintainable micro-optimizations in 
the code unless you can show a measurable performance improvement of the 
kernel.

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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