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

[Rickard Bäckman]

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).

> 
> 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

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
>>>> 
>>