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

Rickard Bäckman rickard.backman at oracle.com
Thu Oct 11 22:06:08 PDT 2012 

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
>>>>>>> 
>>>>> 
>>> 
>

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