Here's a preliminary experiment to see how much memory is saved with the new copy-on-write friendly (bitmap marking) GC.
Calculated by memstats.rb https://gist.github.com/kenn/5105061 on Debian x86_64.
# ./memstats.rb 20547 Memory Summary: private_clean 448 kB private_dirty 24,492 kB pss 28,278 kB rss 73,144 kB shared_clean 4,520 kB shared_dirty 43,684 kB size 184,120 kB swap 0 kB
# ./memstats.rb 20554 Memory Summary: private_clean 0 kB private_dirty 25,012 kB pss 27,695 kB rss 71,216 kB shared_clean 2,648 kB shared_dirty 43,556 kB size 255,504 kB swap 0 kB
rss represents the physical memory that is actually used, and it's comprised of private_clean + private_dirty + shared_clean + shared_dirty.
For the worker process, among 71,216 KB of physical memory, 46,204 KB is shared and 25,012 KB is private, that is, 65% of its memory is shared between workers and the master.
In other words, the footprint of a unicorn worker on this system is only 25MB instead of 70MB. I think it's a big deal!