Re: On the cost of libraries

[Maciej Stachowiak <mjs noisehavoc org>]

On 01Sep2001 05:09PM (-0400), Alex Larsson wrote:
> 
> No app will use all the symbols in the library though, and the 
> measurements I made are worst case (all symbols are found in the last 
> library). In reality most symbols referenced are probably in the lower 
> layers (fan-out effect as higher level functions call lower lever), and 
> lower level libraries are (hopefully) linked earlier. 
> 

Actually, lower-level libraries are typically later on the link line
so they can resolve symbols in the higher-level libraries.

> Note that this only discussed one aspect of libraries, the splitting 
> libraries aspect. Libraries do have other costs too, such as paging in 
> code and fixups. Most of the fixup cost can be fixed by using ELF 
> prelinking, and page-in costs could be lowered with a grope-like tool.

So how come prelinking doesn't help with the symbol lookup cost? You
would think a prelinked binary could know what symbol is in what
shared library, and at what offset, assuming none of the libraries
have changed since the prelinking was done.

> To summarize, I think our current model is good and adding libraries 
> is not a huge problem for us, although I would recommend against random 
> splitting of libraries (i.e. the 10 guppy libraries linked to by gnucash 
> might serve as a bad example).

Thanks for actually running the numbers. This has been realy helpful.

 - Maciej

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