> I wonder how to find how much postgresql memory consumption > is (real, not allocated) ?
You could install the pg_buffercache contrib and
SELECT pg_size_pretty( CAST(current_setting('block_size') AS integer) * max(bufferid) ) FROM pg_buffercache WHERE relfilenode IS NOT NULL;
to find out how much of the shared memory in the buffer pool is actually in use.
There is some extra shared memory that is used for other purposes, and there is private memory of the backends (the latter could be a significant number).
Why do you want that number? If it is for tuning of shared_buffers, that is probably a bad way. Even if shared_buffers is set much higher than you'd actually need it, sooner or later all of your shared_buffers will be used, unless your database is tiny.
