Archie,
Thank you for the explanation, I'm appreciated most of your suggestion,
because mostly I planned to do the same thing as part of interruption
and asynchronized close implementation. But actually the problem is how
Thread can understand what the other thread is blocking on. Please see
my comments inline:
Archie Cobbs wrote:
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 c, 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.
Yes I noticed that select(2) on Linux has this good feature, but I
cannot find similar way on Windows:(.
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.
Yes, that's what I tried to do in the InterruptAction which is given to
Thread.setInterruptAction(). The problem here is not *how* to close the
file descriptor, but is if thread B want to interrupt thread A, the B
don't know *if* there are any file descriptor to be closed for A or
*which* file descriptor to close.
Further, as I mentioned in situation c above, even Thread B can find
Thread A is blocking on some I/O and can know the file descriptor, I'm
not sure how it can know *whether* the I/O is interruptible in Java, for
example, for ServerSocket and ServerSocketChannel which share same fd,
if you invoke ServerSocketChannel.accept(), it is interruptible, but
ServerSocket.accept() is not(at least on RI).
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
This is actually what I did in Harmony-41 for Selector's wakeup()
method, I open a Pipe and add it to Selector's keys set, write a byte to
Pipe.SinkeChannel when wake up.
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).
I'm afraid #1 may not workable in other platforms in Windows, although
this is good feature for Linux. And even it works, the issue above still
exists.
#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.
The other problem is we may not have too many freedom to design the
interruption and Async close implementation, because Java spec requires
the related operation are encapsulated in AbstractInterruptibleChannel's
begin()/end() method, which mark the start/end of some interruptible I/O
operation, so that its subclass can get the capability easily by invoke
the two methods following spec. And AbstractInterruptibleChannel is
extensible to classlib users.
-Archie
__________________________________________________________________________
Archie Cobbs * CTO, Awarix *
http://www.awarix.com
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Paulex Yang
China Software Development Lab
IBM
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