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  Direct buffers are no longer page aligned by default on Java 1.7  (Read 3150 times)
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Offline gouessej
« Posted 2012-07-22 07:58:16 »

Hi

I have just read that direct buffers are no longer page aligned on Java 1.7:
http://download.java.net/jdk8/docs/technotes/guides/io/enhancements.html#jdk7

Is it possible to get the same behavior on Java 1.6 by using -XX:+PageAlignDirectMemory=false? Does it have a bad impact on performance when using big direct NIO buffers? Best regards.

Offline Riven
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« Reply #1 - Posted 2012-07-22 08:06:08 »

FYI: the buffers are now 16 byte aligned, probably with (future) SIMD in mind.


Why would it affect performance? Very few applications care about memory pages. At worst it means 1 more cache miss when iterating through a buffer.

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Offline gouessej
« Reply #2 - Posted 2012-07-22 10:22:58 »

The tech notes don't talk about any improvement for the big NIO buffers and there is still a flag to revert this change so I assume there might be a need of page alignment.

Do you know if -XX:+PageAlignDirectMemory=false works on Java 1.6? I don't want to switch to Java 1.7 just to benefit of this improvement.

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Offline Riven
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« Reply #3 - Posted 2012-07-22 10:25:00 »

This change was not due to performance improvements. It's solely there to reduce memory overhead.



Do you know if -XX:+PageAlignDirectMemory=false works on Java 1.6?
Ofcourse not.

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Offline gouessej
« Reply #4 - Posted 2012-07-22 10:48:03 »

You're right:
Quote
Buildfile: /home/gouessej/Documents/programmation/java/workspace/tuer/build.xml
run-tuer:
     [java] Unrecognized VM option '+PageAlignDirectMemory=false'
     [java] Could not create the Java virtual machine.

BUILD FAILED
/home/gouessej/Documents/programmation/java/workspace/tuer/build.xml:444: Java returned: 1

Total time: 3 seconds

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