RFR: 8087324: Use semaphores when starting and stopping GC task threads

Stefan Karlsson stefan.karlsson at oracle.com
Thu Jul 2 17:58:37 UTC 2015


On 2015-07-02 19:43, Jon Masamitsu wrote:
>
>
> On 6/29/2015 2:38 AM, Stefan Karlsson wrote:
>> Hi all,
>>
>> "8087322: Implement a Semaphore utility class" has now been pushed, 
>> so I've updated the patch to reflect the changes.
>>
>> http://cr.openjdk.java.net/~stefank/8087324/webrev.01.delta
>> http://cr.openjdk.java.net/~stefank/8087324/webrev.01
> http://cr.openjdk.java.net/~stefank/8087324/webrev.01/src/share/vm/gc/shared/workgroup.hpp.frames.html
>
> Are these used?
>
>   194   void print_worker_started_task(AbstractGangTask* task, uint worker_id);
>   195   void print_worker_finished_task(AbstractGangTask* task, uint worker_id);

No. I'll remove them.

>
>
> Rest looks good.  Just one question (just for my education).
>   
> http://cr.openjdk.java.net/~stefank/8087324/webrev.01/src/share/vm/gc/shared/workgroup.cpp.frames.html
>
>   336 void GangWorker::loop() {
>   337   while (true) {
>   338     WorkData data = wait_for_task();
>   339
>   340     run_task(data);
>   341
>   342     signal_task_done();
>   343   }
>   344 }
>
> Does this allow the same thread to execute more than 1
> task ("data" here)?  Meaning if 2 threads are  requested but
> 1 thread is not scheduled to a cpu, will the other thread
> do both chunks of work?

Yes, that's correct.

Thanks for reviewing!
StefanK

>
> Jon
>
>
>>
>> - The IMPLEMENTS_SEMAPHORE_CLASS define was removed, since all 
>> platforms need to provide a Semaphore implementation.
>>
>> - Removed the need to pass down "max number of workers" to the 
>> Semaphore constructor.
>>
>> - Updated semaphore.hpp include path
>>
>> Thanks,
>> StefanK
>>
>>
>> On 2015-06-12 16:52, Stefan Karlsson wrote:
>>> Hi all,
>>>
>>> The current implementation to distribute tasks to GC worker threads 
>>> often cause long latencies (multiple milliseconds) when the threads 
>>> are started and stopped.
>>>
>>> The main reason is that the worker threads have to fight over the 
>>> Monitor lock when they are woken up from the call to Monitor::wait. 
>>> Another reason is that all worker threads call notify_all when they 
>>> finish a task and there wakes all all sleeping worker threads, which 
>>> will yet again force the worker threads to fight over the lock.
>>>
>>> I propose that we use semaphores instead, so that the worker threads 
>>> don't have to fight over a lock when they are woken up.
>>>
>>>
>>> The patches build upon the following patch which introduces a 
>>> Semaphore utility class. This patch will sent out for review on the 
>>> hotspot-dev, since it affects non-GC parts of the code:
>>> http://cr.openjdk.java.net/~stefank/8087322/webrev.00/
>>> https://bugs.openjdk.java.net/browse/JDK-8087322
>>>
>>>
>>> The first patch that I would like to get reviewed is:
>>> http://cr.openjdk.java.net/~stefank/8087323/webrev.00/
>>> https://bugs.openjdk.java.net/browse/JDK-8087323 - Unify and split 
>>> the work gang classes
>>>
>>> It prepares for JDK-8087324, by separating the generic WorkGang 
>>> implementation from the more elaborate YieldingFlexibleWorkGang 
>>> (CMS) implementation. By having this part as a separate patch, I 
>>> hope it will be easier to review JDK-8087324. The patch changes the 
>>> work gang inheritance from:
>>>
>>> AbstractWorkGang
>>>  WorkGang
>>>   FlexibleWorkGang
>>>    YieldingFlexibleWorkGang
>>>
>>> to:
>>>
>>> AbstractWorkGang
>>>  WorkGang
>>>  YieldingFlexibleWorkGang
>>>
>>> Parts of the FlexibleWorkGang and WorkGang code that is going to be 
>>> used by both concrete work gang classes, has been moved into 
>>> AbstractWorkGang. I've duplicated some code in WorkGang and 
>>> YieldingFlexibleWorkGang, but that code will be removed from 
>>> WorkGang in the following patch.
>>>
>>>
>>> The second patch I'd like to get reviewed is:
>>> http://cr.openjdk.java.net/~stefank/8087324/webrev.00/
>>> https://bugs.openjdk.java.net/browse/JDK-8087324 - Use semaphores 
>>> when starting and stopping GC task threads
>>>
>>> It first simplifies the way we distribute the tasks to the GC worker 
>>> threads. For example, the coordinator thread dispatches a task to a 
>>> specific number of workers, and then waits for all work to be 
>>> completed. There's no risk that multiple tasks will be scheduled 
>>> simultaneously, so there's no need for the sequences number that is 
>>> used in the current implementation.
>>>
>>> The patch contains two task dispatch / thread synchronization 
>>> implementations:
>>>
>>> The first implementation uses Monitors, similar to what we did 
>>> before the patch, but with a slightly lower overhead since the code 
>>> calls notify_all less often. It still suffers from the "thundering 
>>> heard" problem. When the coordinator thread signals that the worker 
>>> threads should start, they all wake up from Monitor::wait and they 
>>> all try to lock the Monitor.
>>>
>>> The second, and the more interesting, implementation uses 
>>> semaphores. When the worker threads wake up from the semaphore wait, 
>>> they don't have to serialize the execution by taking a lock. This 
>>> greatly decreases the time it takes to start and stop the worker 
>>> threads.
>>>
>>> The semaphore implementation is used on all platforms where the 
>>> Semaphore class has been implemented in JDK-8087322. So, on some 
>>> OS:es the code will revert to the Monitor-based solution until a 
>>> Semaphore class has been implemented for that OS. So, porters might 
>>> want to consider implementing the Sempahore class.
>>>
>>> There's also a diagnostic vm option 
>>> (-XX:+/-UseSemaphoreGCThreadsSynchronization) to turn off the 
>>> Semaphore-based implementation, which can be used to debug this new 
>>> code. It's mainly targeted towards support and sustaining engineering.
>>>
>>>
>>> The patches have been performance tested on Linux, Solaris, OSX, and 
>>> Windows.
>>>
>>> The effects of the patch can be seen by running benchmarks with 
>>> small young gen sizes, which triggers frequent and short GCs.
>>>
>>> For example, here are runs from the SPECjvm2008 xml.transform 
>>> benchmark with:
>>> -Xmx1g -Xms1g -Xmn64m -XX:+PrintGC -XX:+UseG1GC -jar SPECjvm2008.jar 
>>> -ikv xml.transform -it 30 -wt 30
>>>
>>> I got the following GC times:
>>>
>>>             Average    Median    99.9 percentile   Max
>>> Baseline: 8.76ms    8.44 ms   25.9 ms 34.7 ms
>>> Monitor:   6.17 ms 5.88 ms   26.0 ms 49.1 ms
>>> Semaphore: 3.43 ms 3.26 ms   13.4 ms           33.4 ms
>>>
>>> If I run an empty GC task 10 times per GC, by running the following 
>>> code:
>>> http://cr.openjdk.java.net/~stefank/8087324/timedTask/
>>>
>>> I get the following numbers to complete the empty GC tasks:
>>>
>>>             Average    Median    99.9 percentile   Max
>>> Baseline: 1.43 ms    0.92 ms   3.43 ms           9.30ms
>>> Monitor:    0.75ms 0.72 ms   1.74 ms           2.78ms
>>> Semaphore: 0.07 ms 0.07 ms   0.17 ms           0.26 ms
>>>
>>>
>>>
>>> The code has been tested with JPRT and our nightly testing suites.
>>>
>>> I've created a unit test to run a small test with both the semaphore 
>>> implementation and the monitor implementation:
>>> http://cr.openjdk.java.net/~stefank/8087324/workgangTest/
>>>
>>> But since we currently don't have code to shutdown worker threads 
>>> after they have been started, I don't want to push this test (or 
>>> clean it up) until we have that in place. I created this bug for that:
>>> https://bugs.openjdk.java.net/browse/JDK-8087340
>>>
>>> Thanks,
>>> StefanK
>>>
>>>
>>>
>>
>

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