RFR: 8221394: Clean up ConcurrentGCThread

[Per Liden]

Hi again,

On 3/27/19 8:35 AM, Per Liden wrote:
> Hi Kim,
> 
> On 3/27/19 5:50 AM, Kim Barrett wrote:
>>> On Mar 25, 2019, at 4:54 AM, Per Liden <per.liden at oracle.com> wrote:
>>>
>>> ConcurrentGCThread currently uses Terminator_lock to coordinate 
>>> shutdown. This is dubious, as this lock is shared by many threads, 
>>> some of which calls notify() rather than notify_all() to signal that 
>>> something happened. It's also potentially inefficient as we might be 
>>> waking up more threads than needed. Each ConcurrentGCThread should 
>>> have it's own monitor for this.
>>>
>>> This patch introduces a separate monitor for each instance of 
>>> ConcurrentGCThread and does some general code cleanups in that class.
>>>
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8221394
>>> Webrev: http://cr.openjdk.java.net/~pliden/8221394/webrev.0
>>>
>>> /Per
>>
>> The Terminator_lock is only waited upon by the thread(s) that are
>> stopping the ConcurrentGCThreads and the WatcherThread.  That's
>> normally only one thread, during the shutdown process in before_exit().
>> And before_exit() has special code to ensure no more than one thread
>> gets that far.  So there's no efficiency issue with waking up more
>> threads than needed.
>>
>> I don't have any objection to changing the notify() calls to
>> notify_all() (including the one by the WatcherThread).  The cases
>> where notify() is correct and better than notify_all() are, in my
>> experience, few and far between, and generally needs some commentary
>> to explain why.
>>
>> But I don't think all these new monitors are needed.  The caller of
>> before_exit() is effectively going to go through a sequence of
>> threads, calling stop() on each, and waiting for each to signal that
>> it has stopped (possibly waiting on the Terminator_lock).
> 
> I think you're right. When I first went through the uses of 
> Terminator_lock I got the sense that several threads could be waiting on 
> it, but I think I was wrong there.
> 
> The other reason why I'd like having a monitor per ConcurrentGCThread is 
> encapsulation. Having ConcurrentGCThread be self contained and not 
> depend on an shared lock (that might be used/abused by some other 
> thread) seems like a good thing. One could argue that the current use of 
> Terminator_lock is an implementation details that has leaked out for no 
> good reason, and the overhead of a monitor per thread is negligible.
> 
>>
>> A possibly useful change would be to split the current stop() into a
>> request and a wait, so that, for example, the implementations of
>> CollectedHeap::stop() could request all and then wait for each, rather
>> than looping on request then wait for each, which might give some
>> improvement to shutdown time.  If we were doing that there would be
>> some benefit to separate monitors, though a shared counter of pending
>> requests would work as well.
> 
> I like that idea. I'll give that a shot and come back.

I started going down this path, but fairly quickly ran out of steam as 
it becomes a much larger thing. Let's save the re-factoring for another day.

To move this review forward, I reverted back to using Terminator_lock, 
and changed the notify() in WatcherThread to be a notify_all().

Diff: http://cr.openjdk.java.net/~pliden/8221394/webrev.1-diff
Full: http://cr.openjdk.java.net/~pliden/8221394/webrev.1

/Per