Java NIO windows implementation

问题:

While working on a project using the the NIO.2 AIO features I looked in the "old" NIO selector implementation and saw that on windows the default select-function is used which does not scale at all on windows due to a bad internal implementation. Everybody knows that on windows IOCP is the only real solution. Of course the callback-on-completion model does not fit into the NIO selector model but does this effectively mean that using NIO on windows is basically not a good idea ?

For instance: The new AIO features include an IOCP implementation.

This is especially true while using the latest Netty framework where support for AIO has been dropped. So Netty is not as fast on Windows as it could be ?

回答1:

NIO.2 uses IOCP

The call tree below demonstrates this for file i/o by featuring "Iocp" in several of the called class names, is from Java 7: NIO.2 File Channels on the test bench.

See also sun.nio.ch.Iocp.java, the "Windows implementation of AsynchronousChannelGroup encapsulating an I/O completion port".

NIO does not make use of IOCP, as it only supports "non-blocking i/o" (selectors), not "asynchronous i/o" (completion handlers) that was only added with NIO.2.

回答2:

I think you are confusing asynchronous with faster. Certainly NIO buffers are faster than serializing the same data that would be in the buffers, but many AIO techniques incur costs and delays that can give synchronous IO an advantage.

There was an article a while back that did some pretty good benchmarking of various IO techniques, and the results were (a bit) surprising. The Netty people probably decided to align with the better performing (blocking) IO models mentioned.

回答3:

The problem with IOCP and Java is that IOCP creates and manages threads. My understanding is that for IOCP to work in Java, the event system actually has to go through the Windows IOCP Thread, then scheduled on the Java ThreadPool for execution. This makes IOCP very very expensive to implement in Java versus C++/C#.

AIO was probably removed from Netty because no one wants to sacrifice 450,000 potential transactions just to use AIO versus NBIO. The transactional performance gap between AIO and NBIO is huge.