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> On Apr 8, 2017, at 9:37 AM, Kirk Pepperdine <[email protected]> wrote: > > >> On Apr 8, 2017, at 6:14 PM, Gil Tene <[email protected]> wrote: >> >> There is a lot in this code to "learn" from. > > Agreed…. > >> You can/should obviously use something like jmh to save you much of the >> trouble of making these mistakes. But if you want to learn about the various >> mistakes that people commonly make when trying to measure stuff, this code >> is a good starting point. It is a classic "how not to micro-benchmark" >> example and a good teaching/learning opportunity. So if we change the title >> from "Why a Java method invocation is slower..." to "Why does measuring like >> this give me the wrong impression that...", and putting aside style >> comments, here are some comments and highlights of likely reasons of why you >> are seeing a bigger number for the time elapsed in your single invocation >> from a second call site compared to the 1,000,000 invocations from the >> earlier call site: >> >> - The first call site does not warm up the method. It warms up the call >> site. "Warmup" is not a proper term fir what you are doing there... The >> method can be (and likely was) inlined into the call site and optimized >> there. >> >> - When the second call site is invoked once, it May not have been inlined >> there yet, and may be calling interpreted or lower tiered optimization >> versions of the method. If excercized enough, the second call site would >> likely inline as well. >> >> Separately interesting: >> >> - Your mySleep won't actually do what you think it does. The entire method >> can be optimized away to nothing after inking at the call site by the JIT >> once the calls to it actually warm up enough, since it has no side effects >> and nothing is done with its return code. > > Well, this won’t happen in OpenJDK because of the return value. The return value "saves" you only as long as the method doesn't get inlined. After it is inlined, the fact that the return value isn't used allows the JIT to kill the entire code... > >> >> - Your while(true) loop in the code and the output don't seem to match. >> Where is rest of the output? Or is this output from an earlier code version >> without the while(true) loop? >> >> - You are accumulating more and more contents into an array list. At some >> point the arraylist will be resized. > > It’s pre-sized… > > I’m going to play with this before commenting on the rest. > > — Kirk > > -- > You received this message because you are subscribed to a topic in the Google > Groups "mechanical-sympathy" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/mechanical-sympathy/E7keRLsVcyk/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > [email protected]. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "mechanical-sympathy" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
