How to ask "how big is it?"

["David L. Andre" <DLA@diamond.s4cc.symbolics.com>]

    Date: Fri, 4 Sep 87 15:05 EDT
    From: Mark H. Shirley <MHS@HT.AI.MIT.EDU>

    I've used the document examiner to search for this, without any luck.  Does
    anyone have a utility that grovels over an object in memory and reports on how
    much storage it occupies?  I found SYS:%STRUCTURE-TOTAL-SIZE, but this doesn't
    chase pointers (e.g. to include all objects accessible in a linked
    datastructure).

You have to define what you're looking for.  If you think about it a
little, you'll see that it's not an easy problem.  (For instance, how
much room does a symbol take up?  5 words?  Or do you chase the pointer
to the package and find out that it points to every other symbol in the
world?  Every application has to define what an "important" pointer is.)

I usually end up writing my own tool for each application.  Just write a
quick function which knows how to map over your database, use a hash
table to detect circularities, and then sum up the %STRUCTURE-TOTAL-SIZE
of everything in the hash table.

Don't get fooled by the following phenomenon with lists:

(%STRUCTURE-TOTAL-SIZE (LOOP FOR I BELOW 1000 COLLECT I)) ==> 2

The reason is left as an exercise.