gf2121 commented on issue #12659:
URL: https://github.com/apache/lucene/issues/12659#issuecomment-1759141669
I write a JMH benchmark to compare reading `vint` vs `msbvint`.
```
Benchmark (size) (valueBit) Mode Cnt Score
Error Units
ReadVLongBenchmark.readVLong 128 7 thrpt 5 29.406 ±
0.148 ops/us
ReadVLongBenchmark.readVLong 128 14 thrpt 5 3.644 ±
0.013 ops/us
ReadVLongBenchmark.readVLong 128 21 thrpt 5 2.416 ±
0.004 ops/us
ReadVLongBenchmark.readVLong 128 28 thrpt 5 2.074 ±
0.005 ops/us
ReadVLongBenchmark.readVLong 128 35 thrpt 5 1.838 ±
0.001 ops/us
ReadVLongBenchmark.readVLong 128 42 thrpt 5 1.958 ±
0.004 ops/us
ReadVLongBenchmark.readVLong 128 49 thrpt 5 1.511 ±
0.012 ops/us
ReadVLongBenchmark.readVLong 128 56 thrpt 5 1.463 ±
0.005 ops/us
ReadVLongBenchmark.readVLong 128 63 thrpt 5 1.299 ±
0.092 ops/us
ReadVLongBenchmark.writeVLongMSB 128 7 thrpt 5 28.622 ±
0.865 ops/us
ReadVLongBenchmark.writeVLongMSB 128 14 thrpt 5 4.479 ±
0.013 ops/us
ReadVLongBenchmark.writeVLongMSB 128 21 thrpt 5 3.647 ±
0.007 ops/us
ReadVLongBenchmark.writeVLongMSB 128 28 thrpt 5 2.434 ±
0.372 ops/us
ReadVLongBenchmark.writeVLongMSB 128 35 thrpt 5 1.885 ±
0.006 ops/us
ReadVLongBenchmark.writeVLongMSB 128 42 thrpt 5 1.516 ±
0.005 ops/us
ReadVLongBenchmark.writeVLongMSB 128 49 thrpt 5 1.271 ±
0.004 ops/us
ReadVLongBenchmark.writeVLongMSB 128 56 thrpt 5 1.095 ±
0.003 ops/us
ReadVLongBenchmark.writeVLongMSB 128 63 thrpt 5 0.961 ±
0.002 ops/us
```
<details>
<summary>Code </summary>
```
package testing;
import org.apache.lucene.store.ByteArrayDataOutput;
import org.apache.lucene.store.DataOutput;
import org.apache.lucene.util.packed.PackedInts;
import org.openjdk.jmh.annotations.*;
import java.io.IOException;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
@BenchmarkMode(Mode.Throughput)
@OutputTimeUnit(TimeUnit.MICROSECONDS)
@State(Scope.Benchmark)
@Warmup(iterations = 3, time = 3)
@Measurement(iterations = 5, time = 3)
@Fork(value = 1, jvmArgsPrepend = {"--add-modules=jdk.incubator.vector"})
public class ReadVLongBenchmark {
private byte[] bytes;
private byte[] msbBytes;
private int written;
@Param({"128"})
int size;
@Param({"7", "14", "21", "28", "35", "42", "49", "56", "63"})
int valueBit;
@Setup(Level.Trial)
public void init() throws Exception {
bytes = new byte[size * (valueBit / 7)];
msbBytes = new byte[size * (valueBit / 7)];
ByteArrayDataOutput out = new ByteArrayDataOutput(bytes);
ByteArrayDataOutput msbOut = new ByteArrayDataOutput(msbBytes);
written = 0;
final long min = (1L << valueBit - 1);
final long max = valueBit == 63 ? Long.MAX_VALUE : (1L << valueBit) - 1;
while (out.getPosition() < bytes.length - 10) {
written++;
long l = ThreadLocalRandom.current().nextLong(min, max);
if (PackedInts.bitsRequired(l) != valueBit) {
throw new IllegalStateException();
}
out.writeVLong(l);
writeMSBVLong(l, msbOut);
}
if (readVLong() != writeVLongMSB()) {
throw new IllegalStateException();
}
}
@Benchmark
public long readVLong() {
int pos = 0;
long res = 0;
for (int iter = 0; iter < written; iter++) {
byte b = bytes[pos++];
long i = b & 0x7F;
for (int shift = 7; (b & 0x80) != 0; shift += 7) {
b = bytes[pos++];
i |= (b & 0x7FL) << shift;
}
res ^= i;
}
return res;
}
@Benchmark
public long writeVLongMSB() {
int pos = 0;
long res = 0;
for (int iter = 0; iter < written; iter++) {
long i = 0L;
while (true) {
byte b = msbBytes[pos++];
i = (i << 7) | (b & 0x7FL);
if ((b & 0x80) == 0) {
break;
}
}
res ^= i;
}
return res;
}
static void writeMSBVLong(long l, DataOutput scratchBytes) throws
IOException {
assert l >= 0;
// Keep zero bits on most significant byte to have more chance to get
prefix bytes shared.
// e.g. we expect 0x7FFF stored as [0x81, 0xFF, 0x7F] but not [0xFF,
0xFF, 0x40]
final int bytesNeeded = (Long.SIZE - Long.numberOfLeadingZeros(l) - 1) /
7 + 1;
l <<= Long.SIZE - bytesNeeded * 7;
for (int i = 1; i < bytesNeeded; i++) {
scratchBytes.writeByte((byte) (((l >>> 57) & 0x7FL) | 0x80));
l = l << 7;
}
scratchBytes.writeByte((byte) (((l >>> 57) & 0x7FL)));
}
}
```
</details>
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