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POWhen does java JIT have wrong optimization?
 

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  1. POWhen does java JIT have wrong optimization?
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    <p>JIT does method optimization (-XX:+PrintCompilation) after 10k invocations and you can configure it with -XX:CompileThreshold. I read the reason not to lower that threshold is the JIT optimization might be wrong or you optimize the infrequently used code. I have a few questions regarding this area:</p> <ol> <li>I think the wrong optimization (ie: on-stack-replacement) is due to lazy classloading of polymorphic methods. But after 3 implementations (I think) are found, JVM just do an index table lookup. Of course, speed suffers if you have more polymorphic impl. Is polymorphic method the only cause or major cause for wrong JIT optimization? If not, what are the others?</li> <li>what if I could force loading all the classes at astartup so JVM can build such index tables upfront, isn't it better to do overall optimization upfront? What's wrong with optimize-all approach? what's the cost if my goal is only speed? </li> <li>comparing with C++, if my source are closed which means no 3rd party lib, just like that low-latency system, is there a way to force optimization upfront to increase performance to be closer to that of c++?</li> <li>Peter Lawrey mentioned in his oracle magazine article that you can kick-in JIT by artificially run enough test data in production to meet the threshold. Doing so seems dangerous in production env and one mishap you will be fired. There must be a better way to do it with min risk. </li> <li>any good reference about this topic (involves java &amp; c++) is greatly appreciated.</li> </ol> <p>Updated: #3. never should expect java faster than c++, just want to be closer.</p>
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    1. COC and C++'s philosophy is, "if you screw up, your code probably won't do what you think it will do". Java/C#/Python/most other languages say, "if you do something wrong, I'll give you a error so you can handle the situation". That *very* fact means that Java inherently does more work than C++ -- and, in performance-sensitive cases, that can make a heck of a **lot** of difference.
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    2. COI doubt that @PeterLawrey really said that but I'll let him comment.
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    3. COYou can look up @ http://www.oraclejavamagazine-digital.com/javamagazine/20120506/?lm=1337117562000&pg=49#pg49
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