While it's possible I've missed something, and I'll go check,
Devel::Size is generally an accurate representation of the
amount of actual memory that perl uses for its internal data
structures. This does not mean that it will give you an answer
anywhere near what top will give you--far from it in some cases.
It can't, for
example, show you the memory that might've been allocated by XS code
and not managed by perl. (The expat XML library does this, as do the
various DBD modules, for example) At some point I may put in support
for a few known modules or libraries (like expat, which some of the XML
modules use) but that's a dodgy thing, as it means Devel::Size needs
grubby details of other people's internals as well as perl's.
It also doesn't show you memory your process (but not perl) may
have allocated but that isn't currently tied up in a perl data
structure. Memory allocations are generally done with a particular
granularity, often a power of two or a factor of four or eight
(depends on the type of request, OS, and processor architecture), and
if your perl has requested a chunk of memory smaller than that amount
it'll get silently rounded up, but the extra won't be usable, though
it is put back on the free list if the used bit is freed.
The C RTL also keeps track of allocated and freed memory, since
freed memory is rarely actually given back to the system. While the RTL
tries to satisfy requests for new memory from the pool of freed
chunks, this isn't always possible, and it's not too tough to
accumulate a lot of little bits of memory that are too small to be of
any use. The more alloc/free cycles you have, the more likely you are
to have a significant fragmentation of your memory pool, which usually
means lots of allocated but unusable memory.
A single big alloc with a
resulting free can also blow your process' memory usage way out, even
if you're no longer actually using the memory--it's on your process'
free list, ready to be allocated and still counting against your
memory size, but Devel::Size won't be able to find it.
And, of course, I may well have blown it somewhere. Wouldn't be the
first time and, like I said, I'll go check and see. I'll also go
update the readme for it so folks know what Devel::Size's numbers are
good for, and what they aren't. A general set of docs on memory allocation is probably also in order.