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<td class="text-paragraph-pattern-container mobile-resize-text " style="padding: 0; border-collapse: collapse; padding: 0 0 10px"> <p style="margin: 10px 0 0; margin-top: 0">That's very interesting, thank you so much for investigating!</p> <p style="margin: 10px 0 0">Now for the real puzzle: I remember writing this with a JMH test to make sure the <tt>ConcurrentServiceBinding</tt> would be at worst of comparable efficiency (I'll explain below why) and back then my results were looking better. Not sure why I didn't merge my JMH tests too, my guess is I didn't want to fight Gradle but the point is that sadly my previous tests are lost now; I wish I could compare our JMH tests now.</p> <p style="margin: 10px 0 0">One thing that I'd have done differently in your JMH tests is to not include the key used for the get() operation as a constant, but move it as a field of the state. I tried this change on your shared tests but my <tt>ConcurrentServiceBinding</tt> doesn't win yet so that's not it.</p> <p style="margin: 10px 0 0">So to explain why I would have been happy enough to have a same-performance alternative implementation:</p> <p style="margin: 10px 0 0">in some larger benchmarks (not micro-benchmarks) I often see events like this when enabling <tt>-XX:+PrintCompilation</tt> :</p> <p style="margin: 10px 0 0"><tt>java.util.concurrent.ConcurrentHashMap::get (162 bytes) made not entrant</tt></p> <p style="margin: 10px 0 0">When this is triggered, we revert to interpreted code for a while, until it's compiled again. Preventing this from happening seems not feasible, so my approach was to write an alternative implementation for this very hot point in Hibernate, as profiling had been highlighting "Service lookup" as a problematic point in terms of CPU usage. Admittedly I thought I could write a reasonably efficient alternative to CHM by taking advantage of the fact we only need concurrency on reads, and on identity keys.. to be fair this seemed to have produced good results, but we've tested only on a limited set of benchmarks.</p> <p style="margin: 10px 0 0">So that's the story, but I can follow your suggestion and remove this if it's no longer of value: your data seems solid. I'd like to verify the impact on the "made not entrant" scenario though, and also it would be very interesting to investigate why it's slower... I guess the JVM will likely have strong ad-hoc optimisations and intrinsics to help the CHM implementation but since our requirements are simpler than the typical CHM usage it seems an interesting challenge.</p> <p style="margin: 10px 0 0">To understand why the <tt>ConcurrentServiceBinding</tt> is not as efficient as I'd have hoped, I run this comparison using your JMH benchmark with different parameters, to use it multithreaded and with the "-prof perfnorm" output, getting this (summarized):</p> <p style="margin: 10px 0 0"> <span class="image-wrap" style=""><img src="https://hibernate.atlassian.net/secure/attachment/35933/35933_Comparison+Table+from+JMH+perfnorm.png" style="border: 0px solid black"></span> </p> <p style="margin: 10px 0 0">I'm definitely not an expert in reading this output, some help would be welcome <img class="emoticon" src="cid:jira-generated-image-static-smile-fcd3c367-0c94-4500-a9d6-e97245e4e695" height="16" width="16" align="absmiddle" alt="" border="0"><br> My interpretation is that there are more loads, more stores (because of the recursion?) , many more branches (because we're walking a binary tree?), and generally why more instructions.</p> <p style="margin: 10px 0 0">Maybe we could polish the implementation to store all known hashcodes in an <tt>int[]</tt> so this can be scanned avoiding the recursion, some branches and the additional references which are caused by following the pointers around the tree.</p> <p style="margin: 10px 0 0">Alternatively.. yes I agree we could delete it and stick with the traditional CHM: I'm just annoyed by the "made not reentrant" and "made zombie" .. we could try to remove the custom map and eventually revert according to experience. I'll ask our performance team if they have time to test this.. </p> </td>
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