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From Archie Cobbs <arc...@dellroad.org>
Subject Re: [Fwd: [classlib][NIO|VMI]Interruptible channel implementation - how to interact with Thread?]
Date Thu, 15 Jun 2006 15:45:09 GMT
Paulex Yang wrote:
> Jimmy, Jing Lv wrote:
>> Archie Cobbs wrote:
>>> Paulex Yang wrote:
>>>> Seems Thread's implementation must be aware of what operation it is 
>>>> blocking on. So I propose the following solution:
>>> I don't think the VM or java.lang.Thread needs to be involved.
>>> First of all, the code performing the blocking operation knows
>>> what kind of operation it is, so when it wakes up abnormally it
>>> can take the appropriate action. This code doesn't necessarily
>>> reside in java.lang.Thread.
>>> In Classpath the java.nio stuff is all implemented in native
>>> libraries without the VM or java.lang.Thread being specially
>>> "aware" of anything. However, classlib's design may be different
>>> enough to need it (I haven't studied it as much as you guys).
>>> E.g., the java.nio native code does invoke Thread.interrupt() and
>>> Thread.interrupted(), but these are normal, API-specified methods.
>>> Might be worth taking a look for some design ideas.
>> Thanks Archie, it sounds interesting :).
>> As I study few about Classpath, I still have a question here. As we 
>> know, there are three states of "blocking" on a thread. One is wait(), 
>> sleep() and so forth, thread class handle them itself, it is easy to 
>> interrupt; and one is blocking on I/O, invoke Thread.interrupt() may 
>> be not enough as it is blocked on a system call, e.g., call read on 
>> socket may not return until it receive something or it is closed. In 
>> this way, the Thread.interrupt should know how to close the socket to 
>> perform the real interruption. The question is: how can the thread 
>> know if it is blocked on wait() or I/O operation currently? I think 
>> this is the reason why Paulex require Thread to be involved. So I'm 
>> very interested in what does Classpath do here to stop I/O operation 
>> without get involved?
> Actually Thread.interrupt() is required to handle four different scenario:
> 1. wait(), join(), etc, throw InterruptException
> 2. blocking I/O, close the channel, and throw ClosedByInterruptException
> 3. blocking select, wake up the selector
> 4. none of above, just set the thread's interrupt status
> So if we don't involve Thread and want to implment scenario 2 and 3, we 
> may find the situation is:
> a. If Thread cannot judge scenario 2/3, so it may think they are both 
> scenario 4, so Thread.interrupt() just set the interrupt status and do 
> nothing else, the I/O operation is still blocking there, we cannot get 
> it actually interrupted.
> b. If Thread can find the thread is blocking somewhere, and it considers 
> all blocking as scenario 1, so the InterruptException is thrown, but 
> considering scenario 3, Selector.select() should be waked up without 
> exception, while our selector only has the end() executed in finally 
> block like below, how does end() catch the thrown InterruptException and 
> handle it silently?
> try {
>     begin();
>     // Perform blocking I/O operation here
>     ...
> } finally {
>     end();
> }
> c. If Thread can magically find the thread is blocking on I/O or select, 
> it may need to set the interrupt status, and make the blocked Java 
> method return with some error, so that the end() can check them. 
> Further, the Thread needs to know if this blocking I/O is 
> "interruptible" in Java, for example, the ServerSocket.accept() and 
> ServerSocketChannel.accept() probably uses same system call, but Thread 
> should know ServerSocket cannot be interrupted while ServerSocketChannel 
> can...I have no any idea how Thread can do this without interaction with 
> NIO channels.
> So, Archie, I'm very interested in how Classpath handle this problem. 
> Would you please help to give more details for it (if no legal concern)?

To be honest I'm not sure how exactly it works, or even that it does (I haven't
played with the nio stuff at all).. I only know that Thread implementations in
Classpath don't have special stuff for NIO channels.

Taking a look at Classpath...

In Classpath, if select(2) returns EINTR, the select just returns normally
(with nothing selected) and then the code checks Thread.interrupted().
If set, it closes and throws the exception as necessary.

Also, on UNIX at least, one thread may close a file descriptor that
another thread is waiting on and the second thread will immediately
wake up with an error. So that case is easy to handle.

So the only hard part is waking up the sleeping thread that you have
interrupted. Once it wakes up, the rest can be handled in Java.

A thread blocking on select() will get EINTR if a signal is received. A thread
can signal other threads (via native code) using pthread_kill(). So one
approach would be for the VM to signal a thread with an otherwise ignored
signal when that thread is interrupted. The only possibilities I see are:

1. Interrupt select(2) with a signal
2. select(2) listens on an additional "secret" file descriptor for reading
     and the VM writes a byte into it
3. select(2) is called with a short timeout, and each time it returns
     with timeout we check Thread.interrupted(), then try again.

#1 is most efficient and simplest, but requires VM participation (not much).
#3 has the advantage of being VM indepdendent, but is less efficient.

I think for now at least #3 is a viable alternative. A timeout like 250ms would
give a quick response time with minimal overhead.


Archie Cobbs      *        CTO, Awarix        *      http://www.awarix.com

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