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    <p><strong>Cpython threads:</strong></p> <ul> <li><p>Each cpython thread maps to an OS level thread (lightweight process/pthread in user space)</p></li> <li><p>If there are many cpython threads executing python code concurrently: due to the global interpreter lock, only one cpython thread can interpret python at one time. The remaining threads will be blocked on the GIL when they need to interpret python instructions. When there are many python threads this slows things down a lot. </p></li> <li><p>Now if your python code is spending most of its time inside networking operations (send, connect, etc): in this case there will be less threads fighting for GIL to interpret code. So the effect of GIL is not so bad.</p></li> </ul> <p><strong>Eventlet/Green threads:</strong></p> <ul> <li><p>From above we know that cpython has a performance limitation with threads. Eventlets tries to solve the problem by using a single thread running on a single core and using non blocking i/o for everything.</p></li> <li><p>Green threads are not real OS level threads. They are a user space abstraction for concurrency. Most importantly, N green threads will map to 1 OS thread. This avoids the GIL problem.</p></li> <li><p>Green threads cooperatively yield to each other instead of preemptively being scheduled. For networking operations, the socket libraries are patched in run time (monkey patching) so that all calls are non-blocking.</p></li> <li><p>So even when you create a pool of eventlet green threads, you are actually creating only one OS level thread. This single OS level thread will execute all the eventlets. The idea is that if all the networking calls are non blocking, this should be faster than python threads, in some cases.</p></li> </ul> <p><strong>Summary</strong></p> <p>For your program above, "true" concurrency happens to be faster (cpython version, 5 threads running on multiple processors ) than the eventlet model (single thread running on 1 processor.). </p> <p>There are some cpython workloads that will perform badly on many threads/cores (e.g. if you have 100 clients connecting to a server, and one thread per client). Eventlet is an elegant programming model for such workloads, so its used in several places.</p>
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