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0 pointszedshaw15y ago0 comments

I used the same test everyone has used for the last 8 years to compare poll vs. epoll performance. I also ran it in order to test a hypothesis then assumed I was wrong then ran it more and presented the information openly so others could try it.

Of course I'm going to keep doing this, but if you say that my test is invalid, then all of the tests people did to justify epoll are invalid.

0 comments

jacquesm15y ago

The tests are perfectly valid, it's the conclusion that is wrong. Of course there is a trade-off, of course you'll find a cross-over point because there is a trade off. The fact that the crossover point is at roughly 60% for your kernel config and machine setup is essentially meaningless, other than that it it at least falls within the expected range (50 to 80% or so).

Your hypothesis is that web servers have the majority of their fds active most of the time, and that's where the problem lies. I've put up the numbers elsewhere in this thread, feel free to do some measurement on your own high traffic websites to come up with more data points.

zedshawOP15y ago

"Your hypothesis is that web servers have the majority of their fds active most of the time,"

Aha! Totally wrong. My hypothesis has not been that at all. You totally didn't even understand the hypothesis, and I stated it very clearly. My hypothesis has always been:

epoll is faster than poll when the active/total FD ratio is < 0.6, but poll is faster than epoll when the active/total ratio is > 0.6.

Nowhere in there do I say that all web servers have the majority of their FDs active. NOWHERE. I say some might, I say who knows, I say we need to go measure, but nowhere do I say anything like what you say.

Confusion15y ago

  My hypothesis has always been:

  epoll is faster than poll when the active/total FD ratio is < 0.6, but poll is
  faster than epoll when the active/total ratio is > 0.6.

That wasn't your hypothesis: that was your intermediate conclusion after the first tests. Your hypothesis was: the common knowledge that epoll yields better performance, and that I should obviously use epoll for Mongrel2, is wrong. [1]

It's obvious he didn't mean 'hypothesis', but 'implicit assumption'. If you implement superpoll, you implicitly assume it will be useful. It will only be useful if actually deployed Mongrel2 servers will have an ATR > 0.6 at least some of the time.

[1] You can replace 'is' by 'may be', if you feel the strong version puts words in your mouth. It doesn't, because the hypothesis for an experiment may also be "The half life of protons is shorter than a trillion years", when I expect to reject that hypothesis. It's not an assertion of your opinion, but a statement of a fact you intend to accept or reject based on the outcome of the experiment.

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