command was killed by system #7

hi, today I run this command (./pgd -f europe-airports.mtx --macro sample_graph.macro)in Ubuntu system, but soon the shell was killed, I think that I have enough memory. If you know why, please let me know, thinks.
there is some information about dataset:
europe-airports: nodes:399, edges:5995, mean_degree:30.050125, mean_path:2.323611 , mean_clustering:0.539272, degree_assortativity: -0.22479390366