Cyril的网志

CN wrote:
> Hi,
>
>   what are the implications of using/dropping -fPIC ?
>
> The direct one which comes to my mind is -fPIC reduces the memory
> footprint of the process. Does this _always_ come at the expense
> of CPU cycles?

PIC code results in more instructions to execute.  However, since the
code is shareable by all proc's that are using it, there's a greater
likelihood of keeping the code in-cache closer to the processor
which means fewer cycles lost on cache misses.

So the extremes are 5-15% performance loss if only one process is
using the code, to huge performance gains if you had a large number
of processes executing the same code and the working set for any
given process completely fit in L1 cache.

The actual effect on any given system will, of course, lie somewhere
in the middle.  Any benchmark must be live code running actual
production workload to have any validity since the cache effects
are what govern the outcome and that is extremely difficult to
simulate.

> What other consequences am I looking at if I drop
> PIC totally from one of my shared libraries ?

The only condition where it's worth considering building a non-PIC
shared library is if the library is used solely by a single application
which loads the library before forking working procs.  Since PHP
meets that condition, it's compiled that way.

Rr