Re: Memory statistics

[Lubos Lunak <l lunak suse cz>]

 /me finds it rather amusing, in a sad way, that desktop developers again have 
to discuss such things. Welcome to the club :-/

On Wednesday 26 April 2006 07:07, Behdad Esfahbod wrote:
> On Tue, 25 Apr 2006, Federico Mena Quintero wrote:
> > But yeah, even if we get down to 4 KB per library, it still becomes a
> > big number when you consider 4 KB * num_libraries * num_processes.
>
> AFAIU, other than prelinking, there's no reason that the GOT
> tables have to be page-aligned and per library.  Or is it?

 The GOT is not page-aligned. It's simply just that as soon as you write once 
to it, one page gets dirty. And given that .got, .data and other writable 
sections are together, and you're 99.999999% (maybe even 100%, I don't know) 
guaranteed to have at least one write somewhere, you're bound to have at 
least one dirty page per library per process.

 GOT (and other writable sections) have to be per library because
a) they may contain data (i.e. already in the binary), e.g. .data may contain 
variables with initial values, in .got some relocations contain initial 
offset and are just adjusted - this could be solved by copying such sections 
to a contiguous block of memory though
b) data in them are referred to using an offset, so the data have to be at a 
certain offset from the code (the PIC base register). If these data were 
moved to another place, I don't see how the code would be able to easily find 
the moved base address. Given that the layout of libraries varies per various 
processes, the moved address would vary as well[*]. I would have to check the 
ELF spec to be 100% sure about this though.

[*] Hmm, thinking of that, mapping all .data etc. at a certain absolute 
address with some mapping table at the beginning could do, at least in 
theory.

-- 
Lubos Lunak
KDE developer
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