Direct buffers are no longer page aligned by default on Java 1.7

[gouessej]

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.

[Riven]

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.

[gouessej]

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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[Riven]

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.

[gouessej]

You're right:

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