Reader's Digest Version: Idle for 30 seconds if you must. Longer than that: Turn it off.
读者文摘解答：30秒之内是可以挂空挡的，如果时间比这个长就关掉发动机。
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Currently, the servers found in most data centres continue to slurp energy even when their processors are idle because the server memory cannot be turned off without affecting performance.
Since you are charged per deployed instance, you are better off combining as many processes together as possible, especially if the processes are usually idle.