Re: [rvm-research] Find holes in a block of SegregatedFreeListSpace

Hallo Steve,

Thank you for the reply. Some context is like this. I added some code to scan the mature space of GenMS for holes in the available list.

GenMS.collectionPhase 
  public final void collectionPhase(short phaseId) {

    if (traceFullHeap()) {
      ...........
      if (phaseId == RELEASE) {
          if (Options.verbose.getValue() > 0)
              Log.writeln("main release");
        matureTrace.release();
       
        msSpace.release();

        holes = 0;
        msSpace.linearScan();
        Log.write("holes: ");
        Log.writeln(holes);

        super.collectionPhase(phaseId);
        return;
      }
    }
  }

SegregatedFreeListSpace.linearScan

  public final void linearScan(boolean print) {
    for (int sizeClass = 0; sizeClass < sizeClassCount(); sizeClass++) {
       
        if (VM.VERIFY_ASSERTIONS) {
            VM.assertions._assert(this.flushedBlockHead.get(sizeClass).isZero());
            VM.assertions._assert(this.consumedBlockHead.get(sizeClass).isZero());
        }
       
        Address block = this.availableBlockHead.get(sizeClass);
        Extent blockSize = Extent.fromIntSignExtend(BlockAllocator.blockSize(blockSizeClass[sizeClass]));
       
        while (!block.isZero()) {
            Address next = BlockAllocator.getNext(block);
            this.auditBlock(block, sizeClass, blockSize);       
            block = next;
          }
      }
    }
  }

SegregatedFreeListSpace.auditBlock
   @Inline
   private void auditBlock (Address block, int sizeClass,
            Extent blockSize) {

    Address end = block.plus(blockSize);     
     
    Address cursor = block.plus(blockHeaderSize[sizeClass]);
    Extent cellExtent = Extent.fromIntSignExtend(cellSize[sizeClass]);
   
    while (cursor.LT(end)) {
      ObjectReference current = VM.objectModel.getObjectFromStartAddress(cursor);
      if (current.isNull() || !isCellLive(current)) {
          holes++;
      }   
      cursor = cursor.plus(cellExtent);
    }
  }
 
And I also did a very simple experiment on it. The experiment code added 20 Integer to a static arrayList. Of course, all these objects will be promoted to mature space after gc. At the end of the code I called 10 System.gc() in a sequence and I also set -X:gc:fullHeapSystemGC=true so that full gc was be triggered. The holes of these 10 gcs are
holes: 0
holes: 1367
holes: 348
holes: 1359
holes: 1011
holes: 1359
holes: 348
holes: 1359
holes: 1011
holes: 1359

This doesn't conform to my expectation. I supposed that the second to the tenth "holes" should be 0 when the first is already 0. And in addition, there seems to be some pattern in the numbers -- 1359 = 1011+348. My guess is that this may relate to the markState of MarkSweepSpace which changes from gc to gc. 

BTW, the version I used is 2.9.2

cheers,

Dingwen




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