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From "Weldon Washburn" <weldon...@gmail.com>
Subject Re: [drlvm][threading] H3289 -- some JVMTI questions
Date Fri, 16 Mar 2007 19:22:56 GMT
On 3/16/07, Salikh Zakirov <Salikh.Zakirov@intel.com> wrote:
> Weldon Washburn wrote:
> > On another topic, do we really have to use Thread.stop() in the
> > implementation of vm_shutdown_stop_java_threads() ?  This seems way
> > too hard.   All we really want to do is give back all the OS
> resources.  We
> > could simply call OS kill on each specific thread, return all memory via
> > "free()", return all file handles, etc.  Note if there was no need to
> allow
> > multiple JVMs in a single address space to come and go, we could
> simplify
> > the above and basically just call process exit().
> HARMONY-3381 (already committed) replaces Thread.stop-like shutdown
> algorithm
> to the following one
> 1) set a shutdown callback, which executes hythread_exit() (which
> delegates to
> either pthread_exit() or ExitThread())
> 2) wait for a little while -- this is a little bit "unscientific" and
> assumes,
> that all actively running threads will reach a callback safepoint within
> that
> "little while". If assumption holds true, the remaining threads are highly
> likely to be unactive threads, i.e. sleeping, waiting or blocked.
> The key about a little while is that if all threads has terminated
> promptly, we
> don't need to wait any longer at all. It is currently implemented using
> hythread_join() with timeout.
> 3) on the third step the remaining threads are terminated using
> hythread_cancel() (which delegates to either of pthread_cancel() or
> TerminateThread()) -- This step is inherently dangerous on Windows, as
> terminated thread may be holding system-wide malloc lock, and subsequent
> free()
> on a main thread will deadlock.  Fortunately, the unscientific assumption
> of the
> step (2) makes this rather unprobable.

Yes, agreed this is dangerous and rather unprobable.  I reluctantly agree
with this approach as we have other, more pressing bugs to fix.  Besides, it
is fairly easy to know if we hit problems in this area since the bug will
occur only during shutdown.

Using pthread_cancel() on linux has its own set of advantages and
> disadvantages. By default, threads are created with cancellability enabled
> and
> in "deferred cancellability" state. It means that pthread_cancel() will
> only
> terminate thread when it reaches an explicit cancellation point, such as
> pthread_cond_wait(). In this way, pthread_cancel() does not suffer from
> the
> malloc-deadlock issue.
> However, deferred cancellability brings its own problems, for example, as
> pthread_mutex_lock is not a cancellation point, a pair of deadlocked
> daemon
> threads will still be alive during VM shutdown, and could probably result
> in
> EINVAL return and subsequent assertion failure if the monitors are
> destroyed in
> the main shutdown thread.
> So, the shutdown algorithm has been improved compared to the former
> exception-based approach, like Thread.stop(), but still is not perfect.
> Eugene and me discussed this and agreed to keep an open eye on shutdown
> bugs.

The shutdown code has definitely been improved.  If it works for the apps we
care about, then its good enough for the time being.

There is still a field for experiments and possible improvements, e.g. using
> asynchronous cancellability or even using pthread_kill instead of
> pthread_cancel to implement immediate thread termination.

Hmm... resource reclaimation is always the worst part of building systems
software.  It will probably take a few iterations to get it right.  Somehow
Linux can core dump a bad process and successfully recover all dangling file
handles, network sockets, page directories, thread info blocks, etc.  Maybe
we look at how OS's do resource recovery for some ideas.

> Salikh Zakirov

Weldon Washburn
Intel Enterprise Solutions Software Division

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