shmem/mmap Q

A while back we talked about exec/shmem/mmap.
I am thinking about messing it up and make some test
around this issues when I get some spare time.

I think I figured out that most memory,
at least the buffers is allocated in shmem.

What are the primary reason for this?
Speed? Footprint? Synchonization/Locking? Other?

       regards,
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Gran Thyni, sysadm, JMS Bildbasen, Kiruna