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From "Yang Yang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-2252) arena allocation for memtables
Date Thu, 25 Aug 2011 18:27:30 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2252?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13091196#comment-13091196
] 

Yang Yang commented on CASSANDRA-2252:
--------------------------------------

SlabAllocator:

    private void tryRetireRegion(Region region)
    {
        if (currentRegion.compareAndSet(region, null))
        {
            filledRegions.add(region);
        }
    }


could you please explain why we need to add them to "filledRegions"? when all the buffers
that share the same region die/become unreachable, shouldn't we just let the region go  and
free memory? , then we should not tie this region in memory through the references starting
from filledRegions . no ??

just to confirm my thoughts, I looked at the HBase implementation:
./src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreLAB.java


  /**
   * Try to retire the current chunk if it is still
   * <code>c</code>. Postcondition is that curChunk.get()
   * != c
   */
  private void tryRetireChunk(Chunk c) {
    @SuppressWarnings("unused")
    boolean weRetiredIt = curChunk.compareAndSet(c, null);
    // If the CAS succeeds, that means that we won the race
    // to retire the chunk. We could use this opportunity to
    // update metrics on external fragmentation.
    //
    // If the CAS fails, that means that someone else already
    // retired the chunk for us.
  }

it does not tie it to a region list .



the current result of tying regions together through the filledRegions is that all regions
(even if those dead ones) still occupy memory.  --- well  if the purpose is to count the size()
held in allocator, should we use weak references?


> arena allocation for memtables
> ------------------------------
>
>                 Key: CASSANDRA-2252
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-2252
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Jonathan Ellis
>            Assignee: Jonathan Ellis
>             Fix For: 1.0
>
>         Attachments: 0001-add-MemtableAllocator.txt, 0002-add-off-heap-MemtableAllocator-support.txt,
2252-v3.txt, 2252-v4.txt, merged-2252.tgz
>
>
> The memtable design practically actively fights Java's GC design.  Todd Lipcon gave a
good explanation over on HBASE-3455.

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