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

[David Holmes]

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.

Minor nit: should be an int rather than jint as these are not Java types.

> 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.

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
>>>
>