Do not use a pseudo random for tree traversal
Motivation: If we make allocateRun/SubpageSimple() always try the left node first and make allocateRun/Subpage() always tries the right node first, it is more likely that allocateRun/Subpage() will find a node with ST_UNUSED sooner. Modifications: - Make allocateRunSimple() and allocateSubpageSimple() always try the left node first. - Make allocateRun() and allocateSubpage() always try the right node first. - Remove randome Result: We get the same performance without using random numbers.
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@ -20,11 +20,7 @@ final class PoolChunk<T> {
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private static final int ST_UNUSED = 0;
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private static final int ST_BRANCH = 1;
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private static final int ST_ALLOCATED = 2;
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private static final int ST_ALLOCATED_SUBPAGE = ST_ALLOCATED | 1;
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private static final long multiplier = 0x5DEECE66DL;
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private static final long addend = 0xBL;
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private static final long mask = (1L << 48) - 1;
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private static final int ST_ALLOCATED_SUBPAGE = 3;
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final PoolArena<T> arena;
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final T memory;
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@ -40,8 +36,6 @@ final class PoolChunk<T> {
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private final int chunkSize;
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private final int maxSubpageAllocs;
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private long random = (System.nanoTime() ^ multiplier) & mask;
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private int freeBytes;
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PoolChunkList<T> parent;
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@ -123,22 +117,13 @@ final class PoolChunk<T> {
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case ST_UNUSED:
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return allocateRunSimple(normCapacity, curIdx, val);
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case ST_BRANCH:
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// Try the right node first because it is more likely to be ST_UNUSED.
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// It is because allocateRunSimple() always chooses the left node.
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final int nextIdxLeft = curIdx << 1;
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final int nextValLeft = memoryMap[nextIdxLeft];
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final boolean recurseLeft;
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switch (nextValLeft & 3) {
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case ST_UNUSED:
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return allocateRunSimple(normCapacity, nextIdxLeft, nextValLeft);
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case ST_BRANCH:
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recurseLeft = true;
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break;
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default:
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recurseLeft = false;
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}
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final int nextIdxRight = nextIdxLeft ^ 1;
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final int nextValRight = memoryMap[nextIdxRight];
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final boolean recurseRight;
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switch (nextValRight & 3) {
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case ST_UNUSED:
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return allocateRunSimple(normCapacity, nextIdxRight, nextValRight);
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@ -149,15 +134,28 @@ final class PoolChunk<T> {
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recurseRight = false;
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}
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if (recurseLeft) {
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long res = branchRun(normCapacity, nextIdxLeft);
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final int nextValLeft = memoryMap[nextIdxLeft];
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final boolean recurseLeft;
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switch (nextValLeft & 3) {
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case ST_UNUSED:
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return allocateRunSimple(normCapacity, nextIdxLeft, nextValLeft);
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case ST_BRANCH:
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recurseLeft = true;
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break;
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default:
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recurseLeft = false;
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}
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if (recurseRight) {
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long res = branchRun(normCapacity, nextIdxRight);
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if (res > 0) {
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return res;
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}
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}
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if (recurseRight) {
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return branchRun(normCapacity, nextIdxRight);
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if (recurseLeft) {
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return branchRun(normCapacity, nextIdxLeft);
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}
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}
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@ -193,7 +191,7 @@ final class PoolChunk<T> {
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return curIdx;
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}
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int nextIdx = curIdx << 1 ^ nextRandom();
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int nextIdx = curIdx << 1;
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int unusedIdx = nextIdx ^ 1;
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memoryMap[curIdx] = val & ~3 | ST_BRANCH;
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@ -207,29 +205,29 @@ final class PoolChunk<T> {
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}
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private long allocateSubpage(int normCapacity, int curIdx, int val) {
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int state = val & 3;
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if (state == ST_BRANCH) {
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int nextIdx = curIdx << 1 ^ nextRandom();
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long res = branchSubpage(normCapacity, nextIdx);
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if (res > 0) {
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return res;
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}
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switch (val & 3) {
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case ST_UNUSED:
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return allocateSubpageSimple(normCapacity, curIdx, val);
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case ST_BRANCH:
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// Try the right node first because it is more likely to be ST_UNUSED.
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// It is because allocateSubpageSimple() always chooses the left node.
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final int nextIdxLeft = curIdx << 1;
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final int nextIdxRight = nextIdxLeft ^ 1;
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return branchSubpage(normCapacity, nextIdx ^ 1);
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}
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long res = branchSubpage(normCapacity, nextIdxRight);
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if (res > 0) {
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return res;
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}
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if (state == ST_UNUSED) {
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return allocateSubpageSimple(normCapacity, curIdx, val);
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}
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return branchSubpage(normCapacity, nextIdxLeft);
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case ST_ALLOCATED_SUBPAGE:
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PoolSubpage<T> subpage = subpages[subpageIdx(curIdx)];
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int elemSize = subpage.elemSize;
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if (normCapacity != elemSize) {
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return -1;
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}
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if (state == ST_ALLOCATED_SUBPAGE) {
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PoolSubpage<T> subpage = subpages[subpageIdx(curIdx)];
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int elemSize = subpage.elemSize;
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if (normCapacity != elemSize) {
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return -1;
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}
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return subpage.allocate();
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return subpage.allocate();
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}
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return -1;
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@ -253,7 +251,7 @@ final class PoolChunk<T> {
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return subpage.allocate();
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}
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int nextIdx = curIdx << 1 ^ nextRandom();
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int nextIdx = curIdx << 1;
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int unusedIdx = nextIdx ^ 1;
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memoryMap[curIdx] = val & ~3 | ST_BRANCH;
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@ -363,11 +361,6 @@ final class PoolChunk<T> {
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return memoryMapIdx - maxSubpageAllocs;
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}
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private int nextRandom() {
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random = random * multiplier + addend & mask;
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return (int) (random >>> 47) & 1;
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}
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public String toString() {
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StringBuilder buf = new StringBuilder();
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buf.append("Chunk(");
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