What's the benefits of multi-processing when we already have mult-threading?

I'm confused whether using multiple processes for a web application will improve the performance. Apache's mod_wsgi provides an option to set the number of processes to be started for the daemon process group. I used fastcgi with lighttpd before and it also had an option to configure the max number of processes for each fastcgi application.

While I don't know how multi-processing is better, I do know something bad about it compared to single-process multi-threading model. For example, logging will be harder to implement in multi-processing scenario (link), especially when you also want log rotating. And since memory can't be shared, if you cache something in memory (the most straightforw开发者_C百科ard way), you have multiple duplicate copies.

Do multiple processes better utilize multi-core computing power, or do they yield higher throughput? Or are they just there for some single threaded applications?

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In the case of Python, or more specifically CPython as used by mod_wsgi, the issue is the Python GIL. Although you may have multiple threads in Python, the global interpreter lock effectively means that only one thread can be running Python code at a time. This means it cannot make use of multiple processors/cores properly on a system. Using multiple processes however does allow you to use all those processors/cores.

That said, for mod_wsgi it isn't all Python code but has a lot of C code and with Apache also being C code. During execution of the C code, the GIL is unlocked by that thread meaning that a thread running in C code can run in parallel to a thread running in Python code. Still not the best one can achieve, but can still make partial use of all the processors/core on your system.

For me details of this in relation to mod_wsgi read:

http://blog.dscpl.com.au/2007/09/parallel-python-discussion-and-modwsgi.html http://blog.dscpl.com.au/2007/07/web-hosting-landscape-and-modwsgi.html

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Multi-processing is less efficient than multi-threading, but it's more resilient to failure. Each process gets its own independent memory space and may be terminated and restarted (recycled) independent of other processes.