Ross,
> Here is the
> problem the amount of memory allocated is very close to being
> 3 times the size of the hash tables. even if I undef
> %Original_Hash twice.
I think you're confusing a few issues here.
Q1) When if ever does Perl return memory back to Windows for other
applications?
A1) For most cases, this is only when the entire perl program ends. There
are ways around this but it's out of scope for this question.
Q2) When, if ever, does Perl return memory back to perl for re-use for other
data structures within the current application?
A2) When *all* references to the data structure are "undef"ed or go out of
scope. In your example %alpha will "keep alive" all of the referenced
objects in the values of %orig. Note reassigning a hash to another hash will
not reduce the memory allocated to this hash. Run this to see how many used
vs allocated "buckets" perl is maintaining in the hash.
%x = ("Cat",1,"Dog",2); print scalar(%x),"\n";
@x{@1..10000}=(1)x10000; print scalar(%x),"\n";
%x = ("Cat",1,"Dog",2); print scalar(%x),"\n";
undef %x; print scalar(%x),"\n";
%x = ("Cat",1,"Dog",2); print scalar(%x),"\n";
Q3) When, if ever, does Perl return memory back to perl for re-use within
the same data structure?
This is a black art know by the mystical gods with hallowed names like Wall,
Christiansen, and Schwartz. There are many wonderful optimisations to make
programs go faster or use less memory. From a programmers point we can't
really assume much more than optimisation than what we know from Q2. So as a
rule only declare your data within the minimum scope required to get the job
done.
my @files=<*.*>;
# Don't declare %data here!
for my $file (@files) {
open IN, $file or die "toasted: opening $file $!";
# Declare %data here instead!
my %data;
while (<IN>) {
chomp;
@details=split/,/;
my $key =shift @details;
$data{$key}=\@details;
}
DoStuff(\%data)
# %data and all objects that it alone refers to
# will be magically garbage collected for reuse by this Perl program.
}
If you want to know exactly how much memory Perl is using for a particular
variable then I'm all out of ideas. I posted a similar question to this list
a year or so ago and came up blank.
http://aspn.activestate.com/ASPN/Mail/Message/922634
HTH
Alistair
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