when persistence enabled,MaxDirectMemorySize = walSegmentSize * 4

[38797715 <38...@qq.com>]

hello team,

doc says:

If you use Ignite native persistence, we recommend that you set the 
MaxDirectMemorySize JVM parameter to walSegmentSize * 4

why?

In addition, for the following configuration and code:

"vmArgs": "-XX:+PrintGCDetails -XX:MaxDirectMemorySize=5m"

ByteBuffer bb = ByteBuffer.allocateDirect(6 * 1024 * 1024);

This code throws the following error:
Caused by: java.lang.OutOfMemoryError: Direct buffer memory

Why isn't ignite affected by the MaxDirectMemorySize parameter?

Re: when persistence enabled,MaxDirectMemorySize = walSegmentSize * 4

[akorensh <al...@gmail.com>]

Hi,
  Make sure that -XX:+DisableExplicitGC is not used to avoid facing
https://bugs.openjdk.java.net/browse/JDK-8142537.

  Also use 
-XX:-HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=..

  to take a heap dump and see where memory is being consumed.
  see:
https://docs.oracle.com/javase/8/docs/technotes/guides/troubleshoot/clopts001.html


  Ignite uses a memory mapped file for WAL operations, which is the likely
place where this direct byte buffer memory allocation is taking place.
  see:
https://ignite.apache.org/docs/latest/persistence/native-persistence#wal-modes

 from doc:   
The memory-mapped file approach is used by default and can be switched off
by setting the IGNITE_WAL_MMAP system property to false.


Thanks, Alex



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