Recently I had some tests about Java MappedByteBuffer. I found that if I continuously map the same file and read it, the time spend in reading getting longer and longer. But if I didn't change the map size, It would be faster than the I use the same map size in the map size variation test.
I hava a file "dataFile" in 1GB which filled with Integers.
private final File dataFile = new File("~/testfile");
private final int intNum = 1024 * 1024 * 1024 / 4; // 1GB Integers
@Test
public void writeFile() throws Exception {
DataOutputStream dos = new DataOutputStream(new BufferedOutputStream(new FileOutputStream(dataFile)));
for (int i = 0; i < intNum; i++) {
dos.writeInt(RandomUtils.nextInt());
}
dos.close();
}
And a method about reading it
// read this dataFile in a loop with fixed map size
private void bufferSizePerformanceTest(final int buffSize) throws Exception {
Stopwatch stopwatch = Stopwatch.createStarted();
FileChannel fc = new RandomAccessFile(dataFile, "r").getChannel();
MappedByteBuffer buffer;
final int readPerLoop = buffSize / 4;
int currentLen = 0;
int readCount = 0;
for (int i = 1; ; i++) {
int i1 = i * buffSize;
if (i1 >= dataFile.length()) {
buffer = fc.map(FileChannel.MapMode.READ_ONLY, currentLen, dataFile.length() - currentLen);
for (int j = 0; j < readPerLoop; j++) {
buffer.getInt();
readCount++;
}
break;
} else {
buffer = fc.map(FileChannel.MapMode.READ_ONLY, currentLen, buffSize);
currentLen = i1;
}
for (int j = 0; j < readPerLoop; j++) {
buffer.getInt();
readCount++;
}
}
fc.close();
// ByteBufferUtil.releaseByteBuffer(buffer);
// System.gc();
System.out.println("readCount : " + readCount + " raf buffer size " + getMBytes(buffSize) + " MB : " + stopwatch.elapsed(TimeUnit.MILLISECONDS));
}
The variation buffSize test
private static int getMBytes(int bytes) {
return bytes / 1024 / 1024;
}
// get the power of 2 by n
private static int getM(int n) {
return (int) (Math.log10(n) / Math.log10(2));
}
@Test
public void testBuffSizeReadPerformance() throws Exception {
System.out.println(ManagementFactory.getRuntimeMXBean().getName());
for (int i = 0; i <= getM(1024); i++) {
Thread.sleep(1000);
bufferSizePerformanceTest((int) (Math.pow(2, i) * 1024 * 1024));
}
}
Variation Outputs:
14071@LiuzhMacbook.local
readCount : 268435456 raf buffer size 1 MB : 122
readCount : 268435456 raf buffer size 2 MB : 133
readCount : 268435456 raf buffer size 4 MB : 29
readCount : 268435456 raf buffer size 8 MB : 35
readCount : 268435456 raf buffer size 16 MB : 38
readCount : 268435456 raf buffer size 32 MB : 124
readCount : 268435456 raf buffer size 64 MB : 241
readCount : 268435456 raf buffer size 128 MB : 456
readCount : 268435456 raf buffer size 256 MB : 1086
readCount : 268435456 raf buffer size 512 MB : 2458
readCount : 268435456 raf buffer size 1024 MB : 4952
The fixed buffSize test:
@Test
public void testBuffSizeReadPerformance2() throws Exception {
System.out.println(ManagementFactory.getRuntimeMXBean().getName());
for (int i = 0; i < 10; i++) {
bufferSizePerformanceTest(1024 * 1024 * 1024);
}
}
Output
14157@LiuzhMacbook.local
readCount : 268435456 raf buffer size 1024 MB : 127
readCount : 268435456 raf buffer size 1024 MB : 111
readCount : 268435456 raf buffer size 1024 MB : 20
readCount : 268435456 raf buffer size 1024 MB : 17
readCount : 268435456 raf buffer size 1024 MB : 23
readCount : 268435456 raf buffer size 1024 MB : 19
readCount : 268435456 raf buffer size 1024 MB : 21
readCount : 268435456 raf buffer size 1024 MB : 22
readCount : 268435456 raf buffer size 1024 MB : 20
readCount : 268435456 raf buffer size 1024 MB : 33
As the 2 tests shows, the time spent in reading with the same buffSize(1024MB) is quite different in 2 tests. The test with fixed buffSize is much faster than the variation test.
My question is: 1. How does this happened, why it will be faster? 2. Does the MappedByteBuffer occupy physical memory? As I see in the ActivityMonitor, it will not occupy physical memory.
Thanks
----- Update -----
The release buffer code:
public static void releaseByteBuffer(ByteBuffer buffer) throws NoSuchFieldException, IllegalAccessException {
Cleaner cleaner = ((DirectBuffer) buffer).cleaner();
cleaner.clean();
}
I don't think the cause of this problem is memory usage. Because it has the same output even if I turn on the release code and gc code. Anyway, if is about the memory usage, I set the loop times to 100 in the second test, it should use more memory than the first test, but it's faster than the first one.
----- Update 2 -----
If I turn the buffSize to decrease instead of increase in test 1, the problem disappear.
@Test
public void testBuffSizeReadPerformance3() throws Exception {
System.out.println(ManagementFactory.getRuntimeMXBean().getName());
for (int i = getM(1024); i >= 0; i--) {
bufferSizePerformanceTest((int) (Math.pow(2, i) * 1024 * 1024));
}
}
Output:
16651@LiuzhMacbook.local
readCount : 268435456 raf buffer size 1024 MB : 101
readCount : 268435456 raf buffer size 512 MB : 187
readCount : 268435456 raf buffer size 256 MB : 31
readCount : 268435456 raf buffer size 128 MB : 30
readCount : 268435456 raf buffer size 64 MB : 36
readCount : 268435456 raf buffer size 32 MB : 37
readCount : 268435456 raf buffer size 16 MB : 37
readCount : 268435456 raf buffer size 8 MB : 32
readCount : 268435456 raf buffer size 4 MB : 44
readCount : 268435456 raf buffer size 2 MB : 34
readCount : 268435456 raf buffer size 1 MB : 55
