RFR S: 7127792 Add the ability to change an existing PeriodicTask's execution interval

Thu Oct 11 22:29:07 PDT 2012

I am ok with this change Rickard.

Thanks also to David Holmes for his great feedback and help on this one.

/Markus

Sent from my iPhone

On 12 okt 2012, at 07:06, Rickard Bäckman <rickard.backman at oracle.com> wrote:

> People,
> 
> I need at least one more reviewer, thanks!
> 
> /R
> 
> On Oct 9, 2012, at 3:00 PM, Rickard Bäckman wrote:
> 
>> David,
>> 
>> thanks for your review!
>> 
>> /R
>> 
>> On Oct 9, 2012, at 2:01 PM, David Holmes wrote:
>> 
>>> On 9/10/2012 9:42 PM, Rickard Bäckman wrote:
>>>> David,
>>>> see inline.
>>>> 
>>>> On Oct 9, 2012, at 1:18 PM, David Holmes wrote:
>>>> 
>>>>> On 9/10/2012 7:36 PM, Rickard Bäckman wrote:
>>>>>> David,
>>>>>> 
>>>>>> thanks for your reply!
>>>>>> I've changed the code according to the suggestions, I've also changed the types in PeriodicTask from being a size_t to being a
>>>>>> jint (see updated webrev for details).
>>>>> 
>>>>> But now the type changes have been pushed out to the task creators. Most task creations pass an int already but BiasedLocking uses a size_t. Just shows how messed up the typing was to begin with.
>>>> 
>>>> Agreed. Two ways of solving it, 1) change the callers to use an int. 2) Do the cast in the constructor (should be safe since we check the possible interval).
>>> 
>>> int -> size_t shouldn't cause a warning so callers currently passing int are ok. So keeping it as size_t in constructor arg and casting to int before storing internally seems okay.
>>> 
>>>>> 
>>>>> Minor nit: should be an int rather than jint as these are not Java types.
>>>> 
>>>> Will fix.
>>>> 
>>>>> 
>>>>>> To prevent the waiting for very long time (which could overflow, etc) when we don't have any active task, I added an extra if
>>>>>> so that if we are waiting while no tasks are available, we reset the time_before_wait and consider time_slept to be zero after sleeping.
>>>>>> That means the first task added will always sleep for the period requested.
>>>>> 
>>>>> Those semantics seem reasonable.
>>>>> 
>>>>> The only thing that concerns me here is the affect of calling real_time_tick(0). I can't quite tell what the profiling code does.
>>>> 
>>>> I could avoid calling that if time_slept = 0
>>> 
>>> Ok. You could skip real_time_tick altogether on zero.
>>> 
>>> I'm generally okay with the approach being taken here, but the changes are disruptive enough that I can't see for sure that existing tasks will be unaffected. I guess time will tell. And let's see what others might spot.
>>> 
>>> Thanks,
>>> David (signing off for the night)
>>> 
>>>> Thanks
>>>> /R
>>>> 
>>>>> 
>>>>> David
>>>>> -----
>>>>> 
>>>>>> Updated webrev:
>>>>>> http://cr.openjdk.java.net/~rbackman/7127792.1/
>>>>>> 
>>>>>> Thanks
>>>>>> /R
>>>>>> 
>>>>>> On Oct 9, 2012, at 9:46 AM, David Holmes wrote:
>>>>>> 
>>>>>>> Sorry Rickard, missed the original RFR :)
>>>>>>> 
>>>>>>> So to be clear here the synopsis concerns changing the period, while the mechanism implemented is for a more general dynamic disenroll / enroll. So changing a period is effected by removing a task and then adding it with the new period.
>>>>>>> 
>>>>>>> And for anyone not reading the fine-print when you dynamically enroll a task its first firing is somewhat arbitrary - somewhere between the time of enrollment and that time plus its period.
>>>>>>> 
>>>>>>> src/share/vm/runtime/thread.hpp
>>>>>>> 
>>>>>>> Can you add a comment:
>>>>>>> 
>>>>>>>  static void stop();
>>>>>>> +   // Only allow start once the VM is sufficiently initialized
>>>>>>> +   // Otherwise the first task to enroll will trigger the start
>>>>>>> +   static void make_startable();
>>>>>>> 
>>>>>>> ---
>>>>>>> 
>>>>>>> src/share/vm/runtime/thread.cpp
>>>>>>> 
>>>>>>> We have a bit of type mixing here:
>>>>>>> 
>>>>>>> - size_t time_to_wait
>>>>>>> - jlong time_slept
>>>>>>> - int remaining = time_to_wait
>>>>>>> 
>>>>>>> I don't understand why Task is using size_t for time intervals. If you make "remaining" a size_t then it will cause issues when you pass it to wait. But I would expect the initialization of "remaining" to cause an unsigned-to-signed conversion warning, so perhaps an explicit cast to silence that.
>>>>>>> 
>>>>>>> 1203     bool status = PeriodicTask_lock->wait(Mutex::_no_safepoint_check_flag, remaining);
>>>>>>> 1204     if (status || _should_terminate) {
>>>>>>> 1205       break;
>>>>>>> 1206     }
>>>>>>> 
>>>>>>> Can you rename status to timedout to make the logic more obvious. Also note that you will potentially return with time_slept still at zero, even though you may have slept for an arbitrary amount of time. That seems wrong as zero will then be passed to "tick".
>>>>>>> 
>>>>>>> 1208     // spurious wakeup of some kind
>>>>>>> 
>>>>>>> This comment is no longer accurate as you may have been woken up due to a change in the task list, I suggest:
>>>>>>> 
>>>>>>>   // Change to task list or spurious wakeup of some kind
>>>>>>> 
>>>>>>> 1213     remaining = PeriodicTask::time_to_wait();
>>>>>>> 1214     if (remaining == 0) {
>>>>>>> 1215         continue;
>>>>>>> 1216     }
>>>>>>> 
>>>>>>> Can you insert a comment before continue:
>>>>>>> 
>>>>>>> // Last task was just disenrolled so loop around and wait until
>>>>>>> // another task gets enrolled
>>>>>>> continue;
>>>>>>> 
>>>>>>> 
>>>>>>> 1218     remaining -= time_slept;
>>>>>>> 
>>>>>>> Again type mixing: subtracting a long from an int. Again a potential warning to get rid of.
>>>>>>> 
>>>>>>> Also in a long running VM perhaps there have been no periodic tasks for many days and then one turns up. The subtraction could wrap and cause remaining to remain positive.  I know you've now documented the uncertainty in the first fire time but this seems somewhat too random. Let's see what others think. :)
>>>>>>> 
>>>>>>> 
>>>>>>> 1329   PeriodicTask_lock->notify_all();
>>>>>>> 
>>>>>>> There is only one thread that waits so notify() will suffice.
>>>>>>> 
>>>>>>> Thanks,
>>>>>>> David
>>>>>>> 
>>>>>>> On 9/10/2012 4:34 PM, Rickard Bäckman wrote:
>>>>>>>> Trying again,
>>>>>>>> 
>>>>>>>> can I have a couple of reviews, please?
>>>>>>>> 
>>>>>>>> /R
>>>>>>>> 
>>>>>>>> On Oct 4, 2012, at 3:01 PM, Rickard Bäckman wrote:
>>>>>>>> 
>>>>>>>>> Hi all,
>>>>>>>>> 
>>>>>>>>> can I please have a couple of reviews on the following change:
>>>>>>>>> 
>>>>>>>>> http://cr.openjdk.java.net/~rbackman/7127792/
>>>>>>>>> http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7127792
>>>>>>>>> 
>>>>>>>>> In short the purpose is to enable tasks to change the interval they are executed in. We
>>>>>>>>> would also like to be able to add (and remove) tasks after the WatcherThread has started.
>>>>>>>>> 
>>>>>>>>> Thanks
>>>>>>>>> /R
> 