RFR: 8221394: Clean up ConcurrentGCThread

[Erik Österlund]

Hi Per,

Looks good.

Thanks,
/Erik

On 2019-03-27 14:25, Per Liden wrote:
> 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