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

[Eric Dumazet <dada1@cosmosbay.com>]

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@cosmosbay.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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