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Do not use a pseudo random for tree traversal

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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.
This commit is contained in:
Trustin Lee 2014-05-30 11:23:46 +09:00
parent 1d0a79e11e
commit ea0eb4fdab
1 changed files with 43 additions and 50 deletions
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93
buffer/src/main/java/io/netty/buffer/PoolChunk.java
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@ -20,11 +20,7 @@ final class PoolChunk<T> {
private static final int ST_UNUSED = 0;
private static final int ST_BRANCH = 1;
private static final int ST_ALLOCATED = 2;
private static final int ST_ALLOCATED_SUBPAGE = ST_ALLOCATED | 1;
private static final long multiplier = 0x5DEECE66DL;
private static final long addend = 0xBL;
private static final long mask = (1L << 48) - 1;
private static final int ST_ALLOCATED_SUBPAGE = 3;
final PoolArena<T> arena;
final T memory;
@ -40,8 +36,6 @@ final class PoolChunk<T> {
private final int chunkSize;
private final int maxSubpageAllocs;
private long random = (System.nanoTime() ^ multiplier) & mask;
private int freeBytes;
PoolChunkList<T> parent;
@ -123,22 +117,13 @@ final class PoolChunk<T> {
case ST_UNUSED:
return allocateRunSimple(normCapacity, curIdx, val);
case ST_BRANCH:
// Try the right node first because it is more likely to be ST_UNUSED.
// It is because allocateRunSimple() always chooses the left node.
final int nextIdxLeft = curIdx << 1;
final int nextValLeft = memoryMap[nextIdxLeft];
final boolean recurseLeft;
switch (nextValLeft & 3) {
case ST_UNUSED:
return allocateRunSimple(normCapacity, nextIdxLeft, nextValLeft);
case ST_BRANCH:
recurseLeft = true;
break;
default:
recurseLeft = false;
}
final int nextIdxRight = nextIdxLeft ^ 1;
final int nextValRight = memoryMap[nextIdxRight];
final boolean recurseRight;
switch (nextValRight & 3) {
case ST_UNUSED:
return allocateRunSimple(normCapacity, nextIdxRight, nextValRight);
@ -149,15 +134,28 @@ final class PoolChunk<T> {
recurseRight = false;
}
if (recurseLeft) {
long res = branchRun(normCapacity, nextIdxLeft);
final int nextValLeft = memoryMap[nextIdxLeft];
final boolean recurseLeft;
switch (nextValLeft & 3) {
case ST_UNUSED:
return allocateRunSimple(normCapacity, nextIdxLeft, nextValLeft);
case ST_BRANCH:
recurseLeft = true;
break;
default:
recurseLeft = false;
}
if (recurseRight) {
long res = branchRun(normCapacity, nextIdxRight);
if (res > 0) {
return res;
}
}
if (recurseRight) {
return branchRun(normCapacity, nextIdxRight);
if (recurseLeft) {
return branchRun(normCapacity, nextIdxLeft);
}
}
@ -193,7 +191,7 @@ final class PoolChunk<T> {
return curIdx;
}
int nextIdx = curIdx << 1 ^ nextRandom();
int nextIdx = curIdx << 1;
int unusedIdx = nextIdx ^ 1;
memoryMap[curIdx] = val & ~3 | ST_BRANCH;
@ -207,29 +205,29 @@ final class PoolChunk<T> {
}
private long allocateSubpage(int normCapacity, int curIdx, int val) {
int state = val & 3;
if (state == ST_BRANCH) {
int nextIdx = curIdx << 1 ^ nextRandom();
long res = branchSubpage(normCapacity, nextIdx);
if (res > 0) {
return res;
}
switch (val & 3) {
case ST_UNUSED:
return allocateSubpageSimple(normCapacity, curIdx, val);
case ST_BRANCH:
// Try the right node first because it is more likely to be ST_UNUSED.
// It is because allocateSubpageSimple() always chooses the left node.
final int nextIdxLeft = curIdx << 1;
final int nextIdxRight = nextIdxLeft ^ 1;
return branchSubpage(normCapacity, nextIdx ^ 1);
}
long res = branchSubpage(normCapacity, nextIdxRight);
if (res > 0) {
return res;
}
if (state == ST_UNUSED) {
return allocateSubpageSimple(normCapacity, curIdx, val);
}
return branchSubpage(normCapacity, nextIdxLeft);
case ST_ALLOCATED_SUBPAGE:
PoolSubpage<T> subpage = subpages[subpageIdx(curIdx)];
int elemSize = subpage.elemSize;
if (normCapacity != elemSize) {
return -1;
}
if (state == ST_ALLOCATED_SUBPAGE) {
PoolSubpage<T> subpage = subpages[subpageIdx(curIdx)];
int elemSize = subpage.elemSize;
if (normCapacity != elemSize) {
return -1;
}
return subpage.allocate();
return subpage.allocate();
}
return -1;
@ -253,7 +251,7 @@ final class PoolChunk<T> {
return subpage.allocate();
}
int nextIdx = curIdx << 1 ^ nextRandom();
int nextIdx = curIdx << 1;
int unusedIdx = nextIdx ^ 1;
memoryMap[curIdx] = val & ~3 | ST_BRANCH;
@ -363,11 +361,6 @@ final class PoolChunk<T> {
return memoryMapIdx - maxSubpageAllocs;
}
private int nextRandom() {
random = random * multiplier + addend & mask;
return (int) (random >>> 47) & 1;
}
public String toString() {
StringBuilder buf = new StringBuilder();
buf.append("Chunk(");