Re: Debian shared libs use far more memory than required

[Stephane Chauveau <stephane@chauveau-central.net>]

Thiemo Seufer wrote:

> Andreas Barth wrote:
> [snip]
>  
>
> > > DEBIAN PACKAGE FROM REPOSITORY:
> > > 11 .rodata      000840cb  000000000021a180  000000000021a180   ...
> > > 21 .data         000233c0  00000000003f1d60  00000000003f1d60  ...
> > >
> > > MY OWN RECOMPILED DEBIAN PACKAGE:
> > > 11 .rodata       000a43ad  00000000001f3180  00000000001f3180  ...
> > > 21 .data         00000748  00000000003f3460  00000000003f3460  ...
> > >
> > > That's 0x0233c0-0x748 = 140KB moved from shared to non-shared
> > >
> > > 140KB of non shared memory per GTK application is HUGE!!!
> > >      
>
> Fortunately it is not as bad as it sounds iff the constant data is
> collated together in larger chunks. The kernel does copy-on-write,
> if a .data page is never written, the memory usage is effectively
> the same.
>  

I checked the content of the .data section in libgtk and the
unexpected data appears to  be composed of all exported
symbols aligned to a multiple of 16.  Obviously a symbol
table of some kind.

Unfortunately, they only seem to be grouped per object file.
Some small non-constant data can be found every few KB.
I made a quick estimation and it seems that writing to those
non-constant data would access about 15 pages or 60K with
a 4K page size. Not as bad as the initial 140KB but still impressive.