同一個 pythong script, 在桌機上Run 沒問題，但放到 Digital Ocean 的 linux server 上遇到大量記憶體的運算時會被中止，會顯示 “Killed”:
下了 free 指令，發現沒有 swap，照下面這篇增加 swap, 然後指令就可以正常執行了，感動。
Linux的 VPS 上設定虛擬記憶體 (Swap)
http://max-everyday.com/2016/05/linux-vps-swap/
相關文章：
DO kills my python operation
https://www.digitalocean.com/community/questions/do-kills-my-python-operation
How To Add Swap on Ubuntu 14.04
https://www.digitalocean.com/community/tutorials/how-to-add-swap-on-ubuntu-14-04
What does ‘killed’ mean?
http://stackoverflow.com/questions/19189522/what-does-killed-mean
Why does my python script randomly get killed?
http://stackoverflow.com/questions/1811173/why-does-my-python-script-randomly-get-killed
