Let's Do Some Science: Zed's Test Stack for poll vs. epoll • Hacker News Board

Re: Let's Do Some Science: Zed's Test Stack for poll vs. epoll

#41

Post by pphaneuf » Thu, Aug 05, 2010, 1:30 AM UTC

zedshaw wrote: post #13

codexon wrote: post #5

Zed is probably right that poll is faster than epoll when all descriptors are active. There's no reason to doubt this since epoll has a callback for every file descriptor, while in poll, the OS only has to fill up an array of flags once per call. http://lwn.net/Articles/14168/ However, I will have to side with jacquesm in saying that regular internet servers will usually benefit from epoll. It is unfathomable that we…

There's nothing in anything I've said that disagrees with your assertion that regular web server (not internet servers) have lots of idle connections. What I've been saying all along is something very simple. If the ATR > 0.6, poll wins. If ATR That means, there's potential gains to be had by using both, and at a minimum you can use both and it won't hurt you very much. Additionally, Mongrel2 isn't a regular server.…

Hey, some idea I remembered (from some old paper): don't go from epoll to poll when the ATR gets too high, go to... NOTHING!

Just read() or write(), as if something told you they were ready! Your code has to be able to handle EAGAIN anyway, right? If you're at 1.0 ATR, you're saving the whole cost of poll or epoll!

I'm actually serious here, BTW. It would need some measurement to figure out the threshold at which this is okay, and if it's close enough to 0.6, I'm guessing you could go straight from epoll to nothing, and take the small (potential) performance hit between 0.6 and 0.X.

Hmm, I'm an idiot: this is basically epoll in edge-triggered mode.

Re: Let's Do Some Science: Zed's Test Stack for poll vs. epoll

#42

Post by pphaneuf » Thu, Aug 05, 2010, 1:33 AM UTC

zedshaw wrote: post #29

jfager wrote: post #25

Earlier quoted context omitted.

What I've been saying all along is something very simple. If the ATR > 0.6, poll wins. If ATR You left out the part where you accused people who advocate using epoll of being bullshitting touts who perpetuate fallacies: "why the touted benefits of epoll and most of the information out there is mostly bullshit because of some falacies people seem to have about epoll (mostly perpetuated by epoll's proponents)." Which y…

The touted benefits of epoll are mostly bullshit, since it was advocated as being O(1) and frequently mentioned as being O(1) and always faster than poll by others. In fact, right after mentioning that I thought epoll wasn't faster the first thing people said was it was O(1). Every person I talked to said it. I have evidence that contradicts both of the main assertions of epoll, so until someone comes up with counter…

I've never perceived epoll as being O(1), always O(N), the big thing being the N being how many events are dispatched, rather than the number of watched fds.

I'm fine with the amount of time taken scaling linearly with the amount of work to do, I'm not cool with it scaling linearly with the number of flowers in the garden. :-)

Re: Let's Do Some Science: Zed's Test Stack for poll vs. epoll

#43

Post by pphaneuf » Thu, Aug 05, 2010, 1:42 AM UTC

gthank wrote: post #27

jacquesm wrote: post #16

Earlier quoted context omitted.

You should read more carefully. 40% was the highest observed, 10% the lowest. Also, I said 'the majority', which is not like making it seem like it is only 10%. So it's not '10%' or '40%', for this particular workload, on average you're hovering somewhere between 25 and 30%, since you seem to prefer the higher number make that 30%. And if you read a bit more you would see that there are some factors that could pull t…

Having followed most of the comments on both these threads, I don't think it's fair to characterize Zed's position as saying "epoll is always bad". I think a better characterization would be: "epoll is not automagically better under every imaginable scenario". You may argue that nobody is making any such claim, but I can tell you that the sections of the blogosphere I skim via HN, reddit, etc. do imply it. Maybe the…

Right on. Myself, I always figured epoll was a bit slower than poll when most/all of the fds are ready all the time, from reading the code.

Now, there's an experiment to figure out what the threshold is, and frankly, it's a bit lower than I had hoped (I would have guessed 0.9, 0.8 at worst), which is exactly why having the actual data is awesome.

Re: Let's Do Some Science: Zed's Test Stack for poll vs. epoll

#44

Post by pphaneuf » Thu, Aug 05, 2010, 2:10 AM UTC

zedshaw wrote: post #33

isaacforce wrote: post #24

Earlier quoted context omitted.

>I'm challenging you to provide the data that apparently is still missing that proves that this was not 'a localhost test'. I think what he was saying in relation to it being not 'a localhost test' is that it isn't a test that touches the network stack, even to the loopback interface -- you seem to be equating localhost with anything that happens on a single local machine regardless of whether it has anything to do w…

Yes, nobody seems to get confounding at all. I'm testing poll vs. epoll on selecting active file descriptors to compare their performance. I'm not testing anything else, not claiming anything else. To then test that with a full on server that has an Amazon EC2 cluster blasting requests with HTTP parsing and serving files would completely confuse the analysis. You don't measure a specific thing by inventing some full…

Your results seem correct to me, even though I'm a bit disappointed at the actual number (I was hoping for 0.8 or 0.9, not 0.6), and the code looks good. I'm meaning to run it for myself, but didn't have the time to do so, I should get on it shortly.

I still of the opinion that a >0.6 ATR means you're fucked anyway (where processing all those fds can't be done as fast as data arrives), but I'm trying to come up with a good small test for that hypothesis (like you did).

Until I do, it's just an opinion/hypothesis, and it's kind of hard to argue about it strongly either way.

Re: Let's Do Some Science: Zed's Test Stack for poll vs. epoll

#45

Post by pphaneuf » Thu, Aug 05, 2010, 2:36 AM UTC

zedshaw wrote: post #12

pphaneuf wrote: post #8

Earlier quoted context omitted.

Adding/removing from the poll() array is more or less free, but with epoll, you have to use epoll_ctl(), so you'd definitely want an hysteresis to avoid bouncing things around too much.

Interesting, you're like the 3rd or 4th person who's mentioned hysteresis.

As in "only the 3rd or 4th" (so I'm possibly clever), or as in "a bunch of people"? I don't know your sample size. :-)