Tuning buffer sizes


Overview


When the typical read/write size is small, using a buffer can make a significant improvement in performance. However when reading/writing large amounts of data, additional buffered doesn't help and can add a small amount of overhead.

Additionally, it can be tempting to assume; the larger the buffer the better. However is appears a more modest buffer size, around the size of your L1 cache could be better. e.g 8 KB to 32 KB.


The test


In the following test I compared using buffered and unbuffered reads and writes to a temporary file on a tmpfs filesystem (ram disk)

Size of
read/write
Unbuffered
Writes
  Buffered
Writes
Unbuffered
Reads
  Buffered
Reads
12 MB/s86 MB/s3 MB/s72 MB/s
24 MB/s165 MB/s6 MB/s147 MB/s
48 MB/s333 MB/s11 MB/s291 MB/s
817 MB/s578 MB/s24 MB/s560 MB/s
1634 MB/s920 MB/s49 MB/s983 MB/s
3267 MB/s1,345 MB/s99 MB/s1,679 MB/s
64133 MB/s1,746 MB/s198 MB/s2,518 MB/s
128254 MB/s2,024 MB/s391 MB/s3,387 MB/s
256463 MB/s2,172 MB/s742 MB/s4,104 MB/s
512798 MB/s2,270 MB/s1,334 MB/s4,549 MB/s
1,0241,270 MB/s2,299 MB/s2,355 MB/s4,752 MB/s
2,0481,789 MB/s2,310 MB/s3,704 MB/s4,923 MB/s
4,0962,287 MB/s2,301 MB/s5,324 MB/s4,859 MB/s
8,1922,589 MB/s2,497 MB/s6,346 MB/s6,142 MB/s
16,3842,534 MB/s2,559 MB/s5,764 MB/s5,697 MB/s
32,7682,591 MB/s2,561 MB/s5,793 MB/s5,723 MB/s
65,5362,613 MB/s2,581 MB/s5,861 MB/s5,883 MB/s
131,0722,580 MB/s2,581 MB/s5,401 MB/s5,405 MB/s
262,1441,918 MB/s1,907 MB/s3,269 MB/s3,262 MB/s
524,2881,749 MB/s1,734 MB/s2,845 MB/s2,851 MB/s
1,048,5761,471 MB/s1,469 MB/s2,501 MB/s2,502 MB/s


The default size of a BufferedInputStream and BufferedOutputStream is 8KB. This is part of the reason why they don't help at this point and larger.


The code


BufferTest.java

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