[Catalyst] server memory usage

[John M. Dlugosz]

> You are right in this case: the pages would be shared just after the fork, but would 
> probably start to get copied individually for each process again as soon as the process 
> starts doing something useful. For perl, which works as some kind of JIT compiler, the 
> script executable code is just data and probably gets rewritten very often, so each 
> process would end with its own set of pages.
>
> I'd bet for a KSM capable machine and OS. You can activate and deactivate it, so 
> benchmarking it with your app would be trivial.
>

I would suppose that loading all the modules needed would generate the internal perl 
representation, and that won't change during execution.  It doesn't get rewritten unless 
it's unloaded, which doesn't happen to named package contents.

It's plausible that multiple fire-ups of the same program would wind up with identical 
pages, if no memory allocation ever depends on timing or anything other than static 
configuration.

In Windows, the executable code of a process is "backed" by the exe file and shares 
physical pages with every process that loaded it.  Paging out just forgets it, and paging 
it reads it from the exe file again.  Would you know if Linux, with its fork/exec model, 
does likewise?

Of course, I know that doesn't help the Perl code, which is data as far as the Kernel is 
concerned.

FWIW, VMWare does its own KSM at a higher level, across virtual machines.  So if that is 
possible it does reduce the real load on the host hardware.  But it doesn't change how I'm 
billed!