/dev/shm size?

[Tom Horsley]

So, I just noticed that /dev/shm is 4 gig on my system
with 8 gig of main memory. Does fedora always just automatically
take half the memory for /dev/shm (I've never seen it say
more than 1% was in use, so I'm not sure why it thinks
it needs half). Does tempfs really grab all that memory
up front? Or is that just some max it can grow to if it
wants? Just curious if it affects the memory available
for my virtual machines...