RFR: 8087324: Use semaphores when starting and stopping GC task threads
Stefan Karlsson
stefan.karlsson at oracle.com
Mon Aug 17 07:12:25 UTC 2015
On 2015-08-14 14:38, Stefan Johansson wrote:
> Hi Stefan,
>
> Nice change, looks really good,
Thanks.
> but I have a few small comments.
>
> In workgroup.hpp:
> 177 GangTaskDispatcher* dispatcher()const {
> Add a space before const.
Fixed.
> ---
>
> In workgroup.cpp:
> 122 // Limit the semaphore value to the number of workers.
> 123 _start_semaphore(new Semaphore()),
> 124 _end_semaphore(new Semaphore())
> Seems like the limit has been lost. I would prefer to have it added,
> but if that's somehow problematic just remove the comment.
Removed the comment.
>
> 140 // Wait for the last worker to signal the coordinator.
> 141 _end_semaphore->wait();
> 142
> 143 // No workers are allowed to read the state variables after
> the coordinator has been signaled.
> 144 _task = NULL;
> 145 _started = 0;
> 146 _not_finished = 0;
> Do we need to set _not_finished to 0 or could we instead assert that
> it already is 0 since that's when _end_semaphore should have been
> signaled.
Changed into an assert.
> ---
>
> I don't need to see a new webrev for the above changes. Reviewed.
Thanks,
StefanK
>
> Thanks,
> Stefan
>
> On 2015-08-13 19:50, Stefan Karlsson wrote:
>> Hi all,
>>
>> Could I get a second review for the patch below (plus the suggested
>> removal)?
>>
>> Thanks,
>> StefanK
>>
>> On 2015-07-02 19:58, Stefan Karlsson wrote:
>>> 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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