Spawn is a command designed to run system commands. When you run spawn, you send it a system command that will be run on its own process, but does not execute any further code within your node process. You can add listeners for the process you have spawned, to allow your code interact with the spawned process, but no new V8 instance is created(unless of course your command is another Node command, but in this case you should use fork!) and only one copy of your node module is active on the processor.
Fork is a special instance of spawn, that runs a fresh instance of the V8 engine. Meaning, you can essentially create multiple workers, running on the exact same Node code base, or perhaps a different module for a specific task. This is most useful for creating a worker pool. While node's async event model allows a single core of a machine to be used fairly efficiently, it doesn't allow a node process to make use of multi core machines. Easiest way to accomplish this is to run multiple copies of the same program, on a single processor.
A good rule of thumb is one to two node processes per core, perhaps more for machines with a good ram clock/cpu clock ratio, or for node processes heavy on I/O and light on CPU work, to minimize the down time the event loop is waiting for new events. However, the latter suggestion is a micro-optimization, and would need careful benchmarking to ensure your situation suits the need for many processes/core. You can actually decrease performance by spawning too many workers for your machine/scenario.
Ultimately you could use spawn in a way that did the above, by sending spawn a Node command. But this would be silly, because fork does some things to optimize the process of creating V8 instances. Just making it clear, that ultimately spawn encompasses fork. Fork is just optimal for this particular, and very useful, use case.
http://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback
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