[Sbcl-devel] SBCL-specific sizeof

[Vsevolod D. <vse...@gm...>]

Hi,

I was looking in the mailing list archives and over the Internet but
couldn't find a reference to any code that allows calculating the byte size
of a particular object (mainly, a structure, array or list) in SBCL memory.
I think, no one will argue that this is a handy tool for the selection of a
particular data structure and its layout when dealing with big in-memory
data sets - its primary purpose would be validating a particular choice
with the real-world numbers.

Another use case might be memory profiling: finding the largest object on
the heap currently.

I've also taken a look into room.lisp, but the code found there doesn't
provide enough substance to easily build such a tool. I saw
how sb-vm::map-allocated-objects could be used as it calculates the bytes
used by the object "meta information", but it doesn't account for object's
contents, i.e. it works similar to sizeof(type) invocation in C.

And what about sizeof(instance) - how can it be implemented in SBCL?

Also, another interesting feature of vm::map-allocated-objects is that for
some objects it doesn't have access to them immediately, but "gains" it
after several seconds:

CL-USER> (defparameter *v* #(1 2 3))
*V*
CL-USER> (sizeof *v*)
NIL
;; after a couple of seconds
CL-USER> (sizeof *v*)
48

where
(defun sizeof (o)
  (sb-vm::map-allocated-objects
   (lambda (obj obj-tag n-obj-bytes)
     (when (eql obj o)
       (return-from sizeof n-obj-bytes)))
   :dynamic))

I'd be grateful for any hints on this matter
Vsevolod

---
Vsevolod Dyomkin
+38-096-111-41-56
skype, twitter: vseloved