Re: [RANDOM] Move two variables to read_mostly section to save memory

From: Eric Dumazet
Date: Sun Dec 16 2007 - 13:38:59 EST


Adrian Bunk a Ãcrit :
On Sun, Dec 16, 2007 at 06:42:57PM +0100, Eric Dumazet wrote:
Adrian Bunk a Ãcrit :
...
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.

You omitted an answer to my main important point...

Let me ask it in a more simple way:

Do you see any space difference at all with gcc 4.2 and CONFIG_CC_OPTIMIZE_FOR_SIZE=y ?



I am using gcc-4.2.1

CONFIG_CC_OPTIMIZE_FOR_SIZE=y makes no difference for me.

$ make defconfig
$ egrep "OPTIMIZE_FOR_SIZE|CONFIG_SMP" .config
CONFIG_CC_OPTIMIZE_FOR_SIZE=y
CONFIG_SMP=y
$ make vmlinux
$ nm -v vmlinux|grep -4 random_read_wakeup_thresh
c057a02c d excluded_id_list
c057a100 d zero_bdi
c057a180 D random_table
c057a300 d input_pool
c057a400 d random_read_wakeup_thresh
c057a404 d random_write_wakeup_thresh
<SAME HOLE HERE>
c057a480 d blocking_pool
c057a580 d nonblocking_pool
c057a680 d rekey_work

After my patch, I still gain 120 bytes.

Please realize that most people now build their kernels with CONFIG_SMP=y, or use a distro one (with CONFIG_SMP=y as well)

Your CONFIG_SYSCTL point is valid and should be addressed by a separate patch set.
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