SizeClasses Class — netty Architecture

Source Code

buffer/src/main/java/io/netty/buffer/SizeClasses.java lines 81–413

final class SizeClasses implements SizeClassesMetric {

    static final int LOG2_QUANTUM = 4;

    private static final int LOG2_SIZE_CLASS_GROUP = 2;
    private static final int LOG2_MAX_LOOKUP_SIZE = 12;

    private static final int LOG2GROUP_IDX = 1;
    private static final int LOG2DELTA_IDX = 2;
    private static final int NDELTA_IDX = 3;
    private static final int PAGESIZE_IDX = 4;
    private static final int SUBPAGE_IDX = 5;
    private static final int LOG2_DELTA_LOOKUP_IDX = 6;

    private static final byte no = 0, yes = 1;

    final int pageSize;
    final int pageShifts;
    final int chunkSize;
    final int directMemoryCacheAlignment;

    final int nSizes;
    final int nSubpages;
    final int nPSizes;
    final int lookupMaxSize;
    final int smallMaxSizeIdx;

    private final int[] pageIdx2sizeTab;

    // lookup table for sizeIdx < nSizes
    private final int[] sizeIdx2sizeTab;

    // lookup table used for size <= lookupMaxClass
    // spacing is 1 << LOG2_QUANTUM, so the size of array is lookupMaxClass >> LOG2_QUANTUM
    private final int[] size2idxTab;

    SizeClasses(int pageSize, int pageShifts, int chunkSize, int directMemoryCacheAlignment) {
        int group = log2(chunkSize) - LOG2_QUANTUM - LOG2_SIZE_CLASS_GROUP + 1;

        //generate size classes
        //[index, log2Group, log2Delta, nDelta, isMultiPageSize, isSubPage, log2DeltaLookup]
        short[][] sizeClasses = new short[group << LOG2_SIZE_CLASS_GROUP][7];

        int normalMaxSize = -1;
        int nSizes = 0;
        int size = 0;

        int log2Group = LOG2_QUANTUM;
        int log2Delta = LOG2_QUANTUM;
        int ndeltaLimit = 1 << LOG2_SIZE_CLASS_GROUP;

        //First small group, nDelta start at 0.
        //first size class is 1 << LOG2_QUANTUM
        for (int nDelta = 0; nDelta < ndeltaLimit; nDelta++, nSizes++) {
            short[] sizeClass = newSizeClass(nSizes, log2Group, log2Delta, nDelta, pageShifts);
            sizeClasses[nSizes] = sizeClass;
            size = sizeOf(sizeClass, directMemoryCacheAlignment);
        }

        log2Group += LOG2_SIZE_CLASS_GROUP;

        //All remaining groups, nDelta start at 1.
        for (; size < chunkSize; log2Group++, log2Delta++) {
            for (int nDelta = 1; nDelta <= ndeltaLimit && size < chunkSize; nDelta++, nSizes++) {
                short[] sizeClass = newSizeClass(nSizes, log2Group, log2Delta, nDelta, pageShifts);
                sizeClasses[nSizes] = sizeClass;
                size = normalMaxSize = sizeOf(sizeClass, directMemoryCacheAlignment);
            }
        }

        //chunkSize must be normalMaxSize
        assert chunkSize == normalMaxSize;

        int smallMaxSizeIdx = 0;
        int lookupMaxSize = 0;
        int nPSizes = 0;
        int nSubpages = 0;
        for (int idx = 0; idx < nSizes; idx++) {
            short[] sz = sizeClasses[idx];
            if (sz[PAGESIZE_IDX] == yes) {
                nPSizes++;