Should thread blocked by java NIO Selector.select() be considered waiting or running

Question

The documentation of selectorObj.select() method states

This method performs a blocking selection operation. It returns only after at l

Answer 1

Normally the select operation invokes the poll(...) system function.

select() provides the kernel with a list of file descriptors that it needs to monitor for read/write/error conditions as well as a timeout value. The kernel registers the process/thread with the associated channel's select function and puts the process/thread to sleep. Once the associated channel is ready or a timer has been expired, the kernel wakes up the registered process/thread. Note that this thread is the kernel thread and not the java application thread.

I do not see a reason for the thread doing select to be in WAIT state (Unless the implementation of the Selector returned by the Selector provider explicitly did a wait in the select() function).

Answer 2

When the thread is blocked in an I/O call, it is still running as far as Java thread is concerned.

Most profilers simply show thread state, which is defined as,

• NEW A thread that has not yet started is in this state.
• RUNNABLE A thread executing in the Java virtual machine is in this state.
• BLOCKED A thread that is blocked waiting for a monitor lock is in this state.
• WAITING A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
• TIMED_WAITING A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.
• TERMINATED A thread that has exited is in this state.

As you can see, thread's WAITING/BLOCKED state has nothing to do with I/O.

Answer 3

It depends on the profiler. JProfiler shows the time when the selector thread is blocked on Selector.select() as "Network I/O". So, the best way to interpret the blocked Selector.select() as "Waiting for data".

Hope this helps.

Regards,

Slava Imeshev

Cacheonix In-Memory Data Grid