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Re: question regarding prelinking (was: (inc. note from dpkg developers) (was:Bug#XXXXXX: (far too many packages) needs rebuilt for prelinking))


On Sun, Jan 19, 2003 at 02:48:16PM +0000, Tom Badran wrote:

> On Sunday 19 Jan 2003 2:00 pm, Emile van Bergen wrote:
> > And each time a page of code in a library contains instructions that
> > must be fixed up at load time for an application, that page cannot be
> > shared among applications. A waste of memory.
> This i believe is incorrect, one a library is in memory, all binaries
> that are loaded that use it have their symbols mapped to the library
> in memory. 

Yes, but I mean something different.

To clarify: suppose there's an application X and an application Y, both
dependending on library L, which in turn depends on library M.

Now, each application maps L at a different address. This doesn't
require instructions in L to be modified (which would prevent sharing
pages), because .so libraries contain position independent code. No
wastage here yet.

But then, notice that L uses library M. And suppose that in process X, M
got mapped at a different address than where M got mapped in process Y.
I see no reason for that to be impossible or unlikely, if applications X
and Y each map a different set of libraries before mapping L.

That would mean that all instructions in L that refer to functions or
data in library M, will have to be fixed up using different addresses,
for the applications X and Y. That means that the pages of L containing
those instructions, can suddenly no longer be shared among X and Y.

Please correct me if I'm wrong here, because if I'm right, it would mean
that using lots of interdependent shared libraries is not only slow, it
also needs extra per-process private pages, which means wasting
(potentially lots of) memory.



E-Advies / Emile van Bergen   |   emile@e-advies.info
tel. +31 (0)70 3906153        |   http://www.e-advies.info

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