Re: [PATCH] deinline a few large functions in vlan code v2

[Denis Vlasenko <vda@ilport.com.ua>]

On Wednesday 12 April 2006 20:18, Dave Dillow wrote:
> > > or loaded. And even if it saves 200 bytes in one 
> > > module, unless that module text was already less than 200 bytes into a
> > > page, you've saved no memory -- a 4300 byte module takes 2 pages on x86,
> > > as does a 4100 byte module.
> > 
> > Sometimes, those 200 bytes can bring module size just under 4096.
> > Thus on the average, on many modules you get the same size savings
> > as on built-in code. (Not that we have THAT many network modules...)
> 
> You're making a bogus leap from "sometimes" to "average".

It's not bogus. See below.

> Assuming an even distribution of lengths mod 4096, less than 5% of the
> time will 200 bytes save any memory on a module.

Ok, imagine perfect module size distribution, and suppose we shave 204
bytes off each module. 5% of the modules will have their size reduced
so that they occupy one 4k page less.

Those 5% of modules will have their RAM footprint reduced not
by just 204 bytes, but by 4096 bytes.

Total RAM footprint of all modules, in kb (N=number of modules):
before: sum_of_orig_module_size
after, modular: sum_of_orig_module_size - 0.05*N * 4096
after, builtin: sum_of_orig_module_size - N * 204

0.05*4096 = 204.53

> I don't like "VLAN_ENABLED" as a global define -- it looks too much like
> something local. The CONFIG_VLAN... defines were more descriptive.

That will require Kconfig changes. Maybe you would like "VLAN_ENABLED_KERNER"
name? It doesn't sound local...

> I didn't think about this before, but I'm pretty sure you're taking away
> functionality. When I wrote the typhoon driver, ISTR that I looked
> through the vlan implantation, and determined that all the #ifdefs on
> CONFIG_VLAN_8021Q were not really needed, since all the hooks were
> there. You could just load the 8021q module (even perhaps building it at
> a later date), and it would work if you had filled in the hooks in
> struct net_device. So I didn't #ifdef out code in my driver to let the
> user have the option.

IOW: currently most of VLAN code is already in kernel.
Then why do we have VLAN as a config option? Let's make it unconditional
(will add only 10k to core kernel)?

# size net/8021q/8021q.o
   text    data     bss     dec     hex filename
   9379     484     136    9999    270f net/8021q/8021q.o

> You're taking that away in the name of a total of 5K, which most users
> won't actually get back?

It started as a kernel-wide audit for huge inlines, I was not aiming
at VLAN particularly. But yes, when I saw other (not related to inlines)
opportunities to make code smaller, I decided to do it.

You know, people say that even Linux is getting fat these days.
--
vda
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