On 12 Apr 2011, at 13:51, Jorge Gonzalez wrote:
Rather, it needs to load all the stuff and _then_ fork, so that the
stuff is identical and shared.
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.
Not quite.
What happens is that the perl code is all compiled, and will stay
static for the duration of the program.
Ergo loading a load of stuff, then forking, _will_ give you some
memory sharing.
However, the difficulty is that you don't get to in any way assign
which parts of the memory get perl code (which doesn't change), and
which get variables (which do change). And the operating system's
granularity of 4k on memory pages, means that if you change any 1 byte
within each 4k page, then that page will get unshared.
Ergo even though most of a memory page may contain static code, it'll
be unlucky enough to become unshared if a few bytes are allocated to
variables...
So memory sharing starts out high if you pre-load everything, but
falls off over time..
Cheers
t0m
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