AsynchronousFileChannel.open seems to miss AsynchronousChannelGroup

[Alan Bateman]

On 15/12/2011 12:48, Maxim Mossienko wrote:
> Hi, All
>
> AsynchronousFileChannel.open() seems to miss AsynchronousChannelGroup
> as parameter, the method accepts only ExecutorService.
> When I invoke AsynchronousFileChannel.open with my own executor
> service for many files, each time a new Iocp (AsynchronousChannelGroup
> group implementation on Windows) instance is created with one thread
> per request. If I read 1000 files, this will create 1000 of
> short-lived threads. For such bulk file operation I created
> AsyncChannelGroup with fixed thread pool and see no way to use it. It
> looks like API problem.
>
> Best, Maxim Mossienko
I can't tell if AsynchronousFileChannel is the right choice for your 
specific usage - are you reading these 1000 files sequentially or 
reading from arbitrary positions, perhaps concurrently on each file?

In any case, AsynchronousFileChannel cannot be associated with an 
AsynchronousChannelGroup as there is no guarantee that an implementation 
could service both network and file events with the same I/O facility. 
You may ask why it couldn't use two different I/O mechanisms under the 
covers but this raises questions as to how threads should be split 
between them. So for sharing resources between AsynchronousFileChannel 
implementations then you either use the default thread pool (by invoking 
the open method that doesn't specify a thread pool) or you specify a 
thread pool and the thread pool will be shared.

As regards the one thread you are seeing, this is an internal thread 
used to service the I/O completion port. The reason for it is because 
the thread pool you provide to AsynchronousFileChannel is unknown and so 
the implementation cannot make the assumption that it can use one of the 
threads to service events. If you had 1000 instances with a fixed thread 
pool of 1 for example then 999 ports would not be serviced. That said, 
we could potentially do a better implementation so that when the same 
thread pool is used with several AsynchronousFileChannel instances that 
they share the I/O completion thread and the internal thread. That would 
likely help with your case.

-Alan