Here’s the command, I want to get the average memory usage of one process which pid is 13853. The result is sampled 20 times in one minutes.

ALL=0;
for i in seq 1 20;
do
MEM=pmap 13853|grep total|awk '{print $2}'; ALL=expr$ALL + ${MEM%%K}; sleep 3; done; expr$ALL / 20

The result of pmap is same with VmSize in /proc/status.