Perl fork() shared memory - perl

Question

Is there any way to load read-only variables into memory before forking and keep them there without using up virt memory x number of children?
It seems to be a common opinion that read-only memory is shared by default and is copied on write. I ran a test and found this to be untrue:
#!/usr/bin/perl
my $data;
$$data = 'a'x 1_000_000; #keep it in a ref just in case that matters
foreach (0..10){
last unless my $pid = fork();
}
<STDIN>;
While the process is sitting on STDIN, I check top:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
15982 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15983 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15984 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15985 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15986 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15987 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15988 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15989 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15990 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15991 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
15992 wdev 20 0 121m 2540 188 S 0.0 0.1 0:00.00 foo.t
Sure enough, practically all the memory is in individual child processes.
Is there something I am missing, or does forking a perl process really copy over the entire set of data structures for each child?

Answer 1

Either this is operating-system dependent or you're not seeing what you think you're seeing. I modified your test script to:
#!/usr/bin/env perl
print `free`;
my $data = [ ('a') x 1000000 ];
for (1 .. 10) {
print `free`;
last unless fork;
}
sleep 10;
Running it with ./forktest | grep Mem, I get output such as:
Mem: 3979908 2866552 1113356 0 667388 1258560
Mem: 3979908 2917888 1062020 0 667388 1258560
Mem: 3979908 2918284 1061624 0 667388 1258560
Mem: 3979908 2918532 1061376 0 667388 1258560
Mem: 3979908 2918936 1060972 0 667388 1258560
Mem: 3979908 2919404 1060504 0 667388 1258560
Mem: 3979908 2919900 1060008 0 667388 1258560
Mem: 3979908 2919900 1060008 0 667388 1258560
Mem: 3979908 2920148 1059760 0 667388 1258560
Mem: 3979908 2920148 1059760 0 667388 1258560
Mem: 3979908 2920496 1059412 0 667388 1258560
in which the second column of numbers (third column overall) is the total system RAM usage. Note that it increases from 2866552 to 2917888 when $data is assigned at the start of the program, then stays fairly steady from there on as the forks are done.
I suspect that what you're seeing in top is that it uses "shared memory" in the IPC-specific sense (i.e., chunks of memory which have been explicitly requested and allocated as "shared") and pages which are currently available to multiple processes on a copy-on-write basis do not meet that definition.

Answer 2

The perl docs on fork say "File descriptors (and sometimes locks on those descriptors) are shared, while everything else is copied."