Why is the default char buffer size of BufferedRea

问题:

When I construct a new BufferedReader it is providing me a buffer of 8192 characters. What is the logic/reason behind this?

8192 = 2 to the power of 13

回答1:

Traditionally, memory managers and paging files in the operating system work on pages that are sized in powers of 2. This allows very efficient multiply/divide operations to be performed with left/right shift operations. When working with a buffer, the worst case scenario is to have a buffer with size 1 byte longer than the page size (that would result in an extra page swap with very low benefit). So the default buffer sizes will also tend to be implemented in factors of two.

I'd assume (but have not checked) that the JVM looks for buffers like this and attempts to align them on page boundaries.

Why does this matter? Page misses are quite expensive. If you are doing a ton of IO, it's better to avoid the case where the page backing the buffer gets swapped out to disk (kind of defeats the purpose of the buffer). That said, for most applications, this is a micro-optimization, and for the vast majority of cases, the default is fine.

For reference, Windows and Linux both currently use a 4KB memory page size. So the default buffer on BufferedReader will consume exactly 2 pages.

回答2:

As the BufferedReader Javadoc says

The buffer size may be specified, or the default size may be used. The default is large enough for most purposes.

The default was chosen as being "large enough" (which I would interpret as "good enough").

回答3:

8192, as you said, is 2^13. The exact reason for this number being the default is hard to come by, but I'd venture to say it's based on the combination of normal use scenarios and data efficiency. You can specify a buffer size of whatever you want, though, using a different object constructor.

BufferedReader(Reader in, int sz)

Creates a buffering character-input stream that uses an input buffer of the specified size.

https://docs.oracle.com/javase/7/docs/api/java/io/BufferedReader.html and BufferedReader default buffer size? will provide further insight.