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

Stefan Karlsson stefan.karlsson at oracle.com
Mon Jun 29 09:38:55 UTC 2015

Hi all,

"8087322: Implement a Semaphore utility class" has now been pushed, so 
I've updated the patch to reflect the changes.


- 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 

- Updated semaphore.hpp include path


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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