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Re: eventlet weirdness
First I agree that having blocking DB calls is no big deal given the
way Nova uses mysql and reasonably powerful db server hardware.
However I'd like to point out that the math below is misleading (the
average time for the nonblocking case is also miscalculated but it's
not my point). The number that matters more in real life is
throughput. For the blocking case it's 3/30 = 0.1 request per second.
For the non-blocking case it's 3/27=0.11 requests per second. That
means if there is a request coming in every 9 seconds constantly, the
blocking system will eventually explode but the nonblocking system can
still handle it. Therefore, the non-blocking one should be preferred.
> For example in the API server (before we made it properly multi-threaded) with blocking db calls the server was essentially a serial processing queue - each request was fully processed before the next. With non-blocking db calls we got a lot more apparent concurrencybut only at the expense of making all of the requests equally bad.
> Consider a request takes 10 seconds, where after 5 seconds there is a call to the DB which takes 1 second, and three are started at the same time:
> 0 - Request 1 starts
> 10 - Request 1 completes, request 2 starts
> 20 - Request 2 completes, request 3 starts
> 30 - Request 3 competes
> Request 1 completes in 10 seconds
> Request 2 completes in 20 seconds
> Request 3 completes in 30 seconds
> Ave time: 20 sec
> 0 - Request 1 Starts
> 5 - Request 1 gets to db call, request 2 starts
> 10 - Request 2 gets to db call, request 3 starts
> 15 - Request 3 gets to db call, request 1 resumes
> 19 - Request 1 completes, request 2 resumes
> 23 - Request 2 completes, request 3 resumes
> 27 - Request 3 completes
> Request 1 completes in 19 seconds (+ 9 seconds)
> Request 2 completes in 24 seconds (+ 4 seconds)
> Request 3 completes in 27 seconds (- 3 seconds)
> Ave time: 20 sec
> So instead of worrying about making db calls non-blocking we've been working to make certain eventlets non-blocking - i.e. add sleep(0) calls to long running iteration loops - which IMO has a much bigger impact on the performance of the apparent latency of the system.>>>> Thanks for the explanation. Let me see if I understand this.