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Re: prelink: is it worth it?



On Thu, Jul 9, 2009 at 4:55 PM, Ulrich Drepper <drepper redhat com> wrote:
Adam Miller wrote:
> I am curious as to this answer as well because prelink has been
> something that actually hurt my netbook in performance so I nuked it.

Performance only ever can be hurt because prelink runs periodically to
prelink newly installed code or re-randomize to increase security.

prelink has two benefits:

- almost all relocations a program has to perform are avoided.  These
 can be very expensive when many dependencies and/or large symbol
 tables are involved.  The latter is somewhat mitigated by the new
 symbol table hashing we implemented some time back but still.

- as a side effect of the first point some pages in the loaded binary
 are not copied-on-write.  This can obviously have good effects on
 systems with little memory (netbooks).


Just run your own tests on apps with many relocations and dependencies.
 FF, OO.org, most GUI apps come into mind.  Use

 LD_DEBUG=statistics some/program

to compare numbers.  Run it with and without prelink (but always with
hot disk cache to be fair).  The number of cycles for total startup is
representative of the win.

Note, also small but frequently used apps benefit.  I run gcc etc a lot
and like every single saved cycle.

But something one have to pay a security prize on not disabling it :  it render impossible to have a
centralizzated security integrity management (e.g. rfc.sf.net for example) or one have to skip from check the prelink binary. Very bad i think. 

--
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖


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