In my hands, `top` shows resident memory increasing steadily too....

It is perhaps more instructive to compare output from, for example

seq 1 1000000 | valgrind --time-unit=B --pages-as-heap=yes --trace-children=yes --tool=massif --massif-out-file=massif.dimsum.100000.out.%p dimsum -n 1

with

seq 1 1000000 | valgrind --time-unit=B --pages-as-heap=yes --trace-children=yes --tool=massif --massif-out-file=massif.shuf.100000.out.%p shuf -n 1

in my hands, shuf is faster and uses less memory for this task.

How about you?