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

[Rickard Bäckman]

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