RFR: 8087324: Use semaphores when starting and stopping GC task threads
Jon Masamitsu
jon.masamitsu at oracle.com
Thu Jul 2 15:03:05 UTC 2015
On 07/02/2015 04:48 AM, Stefan Karlsson wrote:
>
>
> On 2015-07-01 18:31, Jon Masamitsu wrote:
>>
>>
>> On 6/12/2015 7:52 AM, 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.
>>
>> http://cr.openjdk.java.net/~stefank/8087323/webrev.00/src/share/vm/gc/cms/yieldingWorkgroup.hpp.frames.html
>>
>> There seems to be only one definition of
>> is_YieldingFlexibleGang_task() now. Is that right? Is that useful?
>>
>> 131 NOT_PRODUCT(virtual bool is_YieldingFlexibleGang_task() const {
>> 132 return true;
>> 133 })
>
> I agree. I don't think we need it anymore.
>
Thanks.
>
>>
>> Not a change in your patch but
>>
>> 86 AbstractWorkGang(const char* name, uint workers, bool are_GC_task_threads, bool are_ConcurrentGC_threads) :
>> 87 _name(name),
>> 88 _total_workers(workers),
>> 89 _active_workers(UseDynamicNumberOfGCThreads ? 1U : workers),
>> 90 _are_GC_task_threads(are_GC_task_threads),
>> 91 _are_Concurren
>>
>> _active_workers is always calculated as >= 2 unless _total_workers is
>> only 1.
>> So line 89 should be
>>
>> _active_workers(UseDynamicNumberOfGCThreads ? MIN2(2, workers) : workers)
>>
>> Should I file a CR for that? Or do you want to include it.
>
> I'm not sure that what is proposed above is correct. I see that
> AdaptiveSizePolicy::calc_active_workers returns 2 as a minimum, but
> both ConcurrentMark::calc_parallel_marking_threads and
> AdaptiveSizePolicy::calc_active_conc_workers can return 1.
>
> I also don't think it should be AbstractWorkGang's responsibility to
> have the knowledge about the minimum number of worker threads that are
> used when UseDynamicNumberOfGCThreads are turned on. Maybe we should
> set it to 0, and let the calc_*_active_workers setup the default value.
I think that at one time I had tried to set the default to 0 and
something failed. I can see the point though.
>
> I would prefer to handle any changes, to this part of the code, as
> separate RFEs.
Fair enough. If I think it's worth doing, I'll file and RFE. Probably
something more than just setting
it to 2 (maybe picking a default value with the help of AdaptiveSizePolicy).
>
>> Have you considered (maybe for a later patch) changing
>> YieldingFlexibleWorkGang to
>> simply YieldingWorkGang? The "Flexible" attribute of
>> YieldingFlexibleWorkGang having
>> been moved into AbstractWorkGang.
>
> I thought about it, but didn't think it was important enough to
> warrant that change in this patch. I wouldn't mind if a RFE was
> created to change the name.
I'll file the RFE if you agree with the point that "Flexible" describes
what AbstractWorkGang does now.
>
>>
>> http://cr.openjdk.java.net/~stefank/8087323/webrev.00/src/share/vm/gc/cms/yieldingWorkgroup.cpp.frames.html
>>
>> Is the cast at 53 necessary? I see it in the original code too.
>>
>> 50 AbstractGangWorker* YieldingFlexibleWorkGang::allocate_worker(uint which) {
>> 51 YieldingFlexibleGangWorker* new_member =
>> 52 new YieldingFlexibleGangWorker(this, which);
>> 53 return (YieldingFlexibleGangWorker*) new_member;
>> 54 }
>
> Yes, this is unnecessary.
Thanks.
Jon
>
>>
>> The rest looks good.
>
> Thanks.
>
>>
>> I'll do the second patch next.
>
> Great.
>
> StefanK
>
>>
>> Jon
>>
>>>
>>>
>>> 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
>>>
>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/hotspot-gc-dev/attachments/20150702/ea42ab57/attachment.htm>
More information about the hotspot-gc-dev
mailing list