fwdptr heap overhead

FwdptrDataModel.java

package org.openjdk.jol.datamodel;

import org.openjdk.jol.util.MathUtil;

public class FwdptrDataModel implements DataModel {

    private final int align;
    private final boolean compressedOops;
    private final FwdptrModel fwdptrModel;

    public FwdptrDataModel(int align, boolean compressedOops, FwdptrModel fwdptrModel) {
        this.align = align;
        this.compressedOops = compressedOops;
        this.fwdptrModel = fwdptrModel;
    }

    @Override
    public int headerSize() {
        return internalHeaderSize(true);
    }

    @Override
    public int arrayHeaderSize() {
        return internalHeaderSize(false) + 4;
    }

    private int internalHeaderSize(boolean alignToOop) {
        // compressed: 8 byte mark + 4 byte class
        // uncompressed: 8 byte mark + 8 byte class
        int headerSize = compressedOops ? 12 : 16;

        switch (fwdptrModel) {
            case No: return headerSize;
            case BeforeObject: return headerSize + align;
            case Uncompressed: return MathUtil.align(headerSize, 8) + 8;
            case Compressed:
                if (alignToOop)
                    return MathUtil.align(headerSize + 4, oopSize());
                else
                    return headerSize + 4;
            default: throw new IllegalStateException();
        }
    }

    @Override
    public int sizeOf(String klass) {
        if (klass.equals("byte"))    return 1;
        if (klass.equals("boolean")) return 1;
        if (klass.equals("short"))   return 2;
        if (klass.equals("char"))    return 2;
        if (klass.equals("int"))     return 4;
        if (klass.equals("float"))   return 4;
        if (klass.equals("long"))    return 8;
        if (klass.equals("double"))  return 8;
        return oopSize();
    }

    public int oopSize() {
        return compressedOops ? 4 : 8;
    }

    @Override
    public int objectAlignment() {
        return align;
    }

    @Override
    public String toString() {
        return "X64 model, " + align + "-byte aligned, with fwdptr";
    }

}

Overheads.txt

Random JAR-files:
       oop ||         compressed        ||         uncompressed      ||
    fwdptr || no   | bef  | unco | comp || no   | bef  | unco | comp ||
===========||======|======|======|======||======|======|======|======||
   align 8 || 1.00 | 1.24 | 1.33 | 1.09 || 1.46 | 1.71 | 1.71 | 1.71 ||
  align 16 || 1.09 | 1.57 | 1.47 | 1.20 || 1.55 | 2.04 | 1.86 | 1.86 ||
  align 32 || 1.36 | 2.34 | 1.62 | 1.43 || 1.83 | 2.80 | 2.04 | 2.04 ||
  
Eclipse:
  
       oop ||         compressed        ||         uncompressed      ||
    fwdptr || no   | bef  | unco | comp || no   | bef  | unco | comp ||
===========||======|======|======|======||======|======|======|======||
   align 8 || 1.00 | 1.17 | 1.23 | 1.06 || 1.30 | 1.47 | 1.47 | 1.44 ||
  align 16 || 1.09 | 1.43 | 1.31 | 1.16 || 1.36 | 1.71 | 1.58 | 1.54 ||
  align 32 || 1.19 | 1.87 | 1.47 | 1.24 || 1.51 | 2.19 | 1.76 | 1.70 ||
  
IntelliJ:

       oop ||         compressed        ||         uncompressed      ||
    fwdptr || no   | bef  | unco | comp || no   | bef  | unco | comp ||
===========||======|======|======|======||======|======|======|======||
   align 8 || 1.00 | 1.12 | 1.16 | 1.05 || 1.22 | 1.33 | 1.33 | 1.31 ||
  align 16 || 1.06 | 1.29 | 1.21 | 1.12 || 1.27 | 1.50 | 1.40 | 1.38 ||
  align 32 || 1.14 | 1.60 | 1.31 | 1.20 || 1.35 | 1.82 | 1.52 | 1.49 ||
  
NetBeans:

       oop ||         compressed        ||         uncompressed      ||
    fwdptr || no   | bef  | unco | comp || no   | bef  | unco | comp ||
===========||======|======|======|======||======|======|======|======||
   align 8 || 1.00 | 1.14 | 1.19 | 1.04 || 1.26 | 1.40 | 1.40 | 1.37 ||
  align 16 || 1.07 | 1.35 | 1.25 | 1.12 || 1.30 | 1.59 | 1.49 | 1.47 ||
  align 32 || 1.16 | 1.73 | 1.39 | 1.18 || 1.43 | 2.00 | 1.63 | 1.60 ||