Demo: Reserving Costs Nothing

Three mmap() calls of 4 MB, three munmap()s, ps at every step.

Run

Two terminals. First:

make
./reserve_mem

Second, at every prompt:

ps -o pid,vsz,rss,cmd -p $(pidof reserve_mem)
pmap -x $(pidof reserve_mem) | grep -E 'anon|total'

Ask

• VSZ moves by 4,096 KB a step. What does RSS do, seven times in a row?
• The three zones have different permissions. Why are they three lines rather than one?
• What did the kernel write down for each mmap(), and what did it not do?