Why is the performance of BufferedReader so much w

问题:

I understand that using a BufferedReader (wrapping a FileReader) is going to be significantly slower than using a BufferedInputStream (wrapping a FileInputStream), because the raw bytes have to be converted to characters. But I don't understand why it is so much slower! Here are the two code samples that I'm using:

BufferedInputStream inputStream = new BufferedInputStream(new FileInputStream(filename));
try {
  byte[] byteBuffer = new byte[bufferSize];
  int numberOfBytes;
  do {
    numberOfBytes = inputStream.read(byteBuffer, 0, bufferSize);
  } while (numberOfBytes >= 0);
}
finally {
  inputStream.close();
}

and:

BufferedReader reader = new BufferedReader(new FileReader(filename), bufferSize);
try {
  char[] charBuffer = new char[bufferSize];
  int numberOfChars;
  do {
    numberOfChars = reader.read(charBuffer, 0, bufferSize);
  } while (numberOfChars >= 0);
}
finally {
  reader.close();
}

I've tried tests using various buffer sizes, all with a 150 megabyte file. Here are the results (buffer size is in bytes; times are in milliseconds):

Buffer   Input
  Size  Stream  Reader
 4,096    145     497
 8,192    125     465
16,384     95     515
32,768     74     506
65,536     64     531

As can be seen, the fastest time for the BufferedInputStream (64 ms) is seven times faster than the fastest time for the BufferedReader (465 ms). As I stated above, I don't have an issue with a significant difference; but this much difference just seems unreasonable.

My question is: does anyone have a suggestion for how to improve the performance of the BufferedReader, or an alternative mechanism?

回答1:

The BufferedReader has convert the bytes into chars. This byte by byte parsing and copy to a larger type is expensive relative to a straight copy of blocks of data.

byte[] bytes = new byte[150 * 1024 * 1024];
Arrays.fill(bytes, (byte) '\n');

for (int i = 0; i < 10; i++) {
    long start = System.nanoTime();
    StandardCharsets.UTF_8.decode(ByteBuffer.wrap(bytes));
    long time = System.nanoTime() - start;
    System.out.printf("Time to decode %,d MB was %,d ms%n",
            bytes.length / 1024 / 1024, time / 1000000);
}

prints

Time to decode 150 MB was 226 ms
Time to decode 150 MB was 167 ms

NOTE: Having to do this intermixed with system calls can slow down both operations (as system calls can disturb the cache)

回答2:

in BufferedReader implementation there is a fixed constant defaultExpectedLineLength = 80, which is used in readLine method when allocating StringBuffer. If you have big file with lots of lines longer then 80, this fragment might be something that can be improved

if (s == null) 
    s = new StringBuffer(defaultExpectedLineLength);
s.append(cb, startChar, i - startChar);