First, note that there are at least two important measures
of memory use: total memory size and working set size.
I tend to oversimplify them slightly and think of them as
"paging file use" and "RAM use", respectively. Total
memory size is how many pages of virtual memory your
program has been allocated. Working set size is how many
pages of memory your program needs to have in RAM at once
in order to run smoothly.
If your program uses more total memory than is available
to it (either due to quota restrictions or just how much
is available to the operating system minus how much the
kernel and other processes are using), it will probably
die [well, usually malloc() will fail, which will
usually either cause a quick death or trigger bugs which
cause a slightly slower but more colorful death; but some
operating systems will let malloc() succeed but when you
actually try to use the memory the OS will notice the
problem and kill you (or some other process!)].
If your program needs a larger working set than is
available, then it will start "paging" too much and slow
down noticeably. The size of the working set has a lot
to with how the memory is accessed. By making your
data structures such that related items are close together
in memory, you can often dramatically reduce your program's
working set size. BTW, Perl plays this same trick by
trying to arrange for its most-used C subroutines to be
close together in memory (which is why we have the
To get the working set size, you'll have to ask the
operating system. To get the total memory size, I'd
build a version of Perl with DEBUGGING_MSTATS defined
and then use Devel::Peek::mstats (though the documentation
for this has been moved or purged so you may have to dig
for it or reverse engineer it).
(but my friends call me "Tye")
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