with the default setting num_cpu= 16 , I ran out of my 40G RAM and process was killed by system.
sudo cat /var/log/syslog | grep -i "killed"
kernel: [ 1522.255350] Out of memory: Killed process 15384 (python) total-vm:54111924kB, anon-rss:35495356kB, file-rss:72320kB, shmem-rss:14336kB, UID:0 pgtables:76780kB oom_score_adj:0