It occurred that if ./mtr -
rr --rr-arg=-chaos is being used for reproducing some bug, then also --no-check-testcase is useful. Basically the reason is that rr record --chaos nproc will return random results. This explains why THREAD_POOL_SIZE would differ according to check-testcase. IMO it would be good to document the usage of https://rr-project.org/ somewhere.