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

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Date:	Sun, 16 Dec 2007 18:42:57 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Adrian Bunk <bunk@...nel.org>
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

Adrian Bunk a écrit :
> On Sun, Dec 16, 2007 at 03:44:37PM +0100, Eric Dumazet wrote:
>> Adrian Bunk a écrit :
>>> 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.
>> You seem to to be confused between speed micro-otimizations and memory 
>> savings. This patch has nothing to do about a speed optimization. Here, no 
>> tradeoff justify a "measurable performance improvement" study.
>>
>> I copied this patch to you because your recent proposal to remove 
>> read_mostly from linux kernel.
>>
>> Only you find read_mostly ugly and unmaintanable. I find it way more 
>> usefull than "static" attributes.
>>
>> I find 120 bytes is a measurable gain, thank you.
> 
> 
> I am well aware that your patch is about space saving and not speed
> improvement.
> 
> But trying to save space this way is simply not maintainable.
> 
> And it's trivial to see that your patch actually makes the code _bigger_ 
> for all people who try hard to get their kernel small and use 
> CONFIG_SYSCTL=n - funnily your patch has exactly the problem I described 
> as drawback of __read_mostly in the thread you are referring to...
> 
> 
> And even more funny, with gcc 4.2 and CONFIG_CC_OPTIMIZE_FOR_SIZE=y your 
> patch doesn't seem to make any space difference - are you using an older 
> compiler or even worse CONFIG_CC_OPTIMIZE_FOR_SIZE=n for being able to 
> see any space difference?
> 
> In both cases your code uglification would be even more pointless...
> 

I believe that CONFIG_SMP is uglification for you Adrian, but still I am glad 
linux have it.

If your CONFIG_SYSCTL=n is really that important for you, why dont you define 
a new qualifier that can indeed mark some variables as :

const if CONFIG_SYSCTL=n
read_mostly if CONFIG_SYCTL=y

This way you can keep compiler optimizations for your CONFIG_SYCTL=n, while 
many people like me can still continue to optimize their kernel.

Seeing so many sysctl already read_mostly in kernel, I wonder why you NACK my 
patch, while it's easy to share your concerns with other people and find a 
solution.

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