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POGiven that HashMaps in jdk1.6 and above cause problems with multi=threading, how should I fix my code
 

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  1. POGiven that HashMaps in jdk1.6 and above cause problems with multi=threading, how should I fix my code
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    <p>I recently raised a question in stackoverflow, then found the answer. The initial question was <a href="https://stackoverflow.com/questions/13969076/what-mechanisms-other-than-mutexs-or-garbage-collection-can-slow-my-multi-thread/14003357#14003357">What mechanisms other than mutexs or garbage collection can slow my multi-threaded java program?</a></p> <p>I discovered to my horror that HashMap has been modifed between JDK1.6 and JDK1.7. It now has a block of code that causes all threads creating HashMaps to synchronize.</p> <p>The line of code in JDK1.7.0_10 is</p> <pre><code> /**A randomizing value associated with this instance that is applied to hash code of keys to make hash collisions harder to find. */ transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this); </code></pre> <p>Which ends up calling</p> <pre><code> protected int next(int bits) { long oldseed, nextseed; AtomicLong seed = this.seed; do { oldseed = seed.get(); nextseed = (oldseed * multiplier + addend) &amp; mask; } while (!seed.compareAndSet(oldseed, nextseed)); return (int)(nextseed &gt;&gt;&gt; (48 - bits)); } </code></pre> <p>Looking in other JDKs, I find this isn't present in JDK1.5.0_22, or JDK1.6.0_26. </p> <p>The impact on my code is huge. It makes it so that when I run on 64 threads, I get less performance than when I run on 1 thread. A JStack shows that most of the threads are spending most of their time spinning in that loop in Random.</p> <p>So I seem to have some options:</p> <ul> <li>Rewrite my code so that I don't use HashMap, but use something similar</li> <li>Somehow mess around with the rt.jar, and replace the hashmap inside it</li> <li>Mess with the class path somehow, so each thread gets its own version of HashMap</li> </ul> <p>Before I start down any of these paths (all look very time consuming and potentially high impact), I wondered if I have missed an obvious trick. Can any of you stack overflow people suggest which is the better path, or perhaps identify a new idea.</p> <p>Thanks for the help</p>
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    1. COWhat requires you to create that many hashmaps? What are you trying to do?
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    2. COthe application is doing a lot of searching. The hashmaps hold the data structure that is being searched, the results of the search, and quite a few other things. It is also about 100 man-years worth of development...so it might be hard to change to something else. The data that is being searched is schemaless so very hard to model in POJOs
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    3. CO2 comments: 1. ConcurrentHashMap does not seem to use that - could it be an alternative? 2. This piece of code is only called on map creation. That implies that you are creating millions of hashmaps under high contention - Does that really reflect a realistic production load?
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