Unexpected issues with Final Mark pauses and pacer performance in JDK11
Mathiske, Bernd
mathiske at amazon.com
Wed Aug 5 18:27:35 UTC 2020
Zhengyu,
Here are GC timings from the GC log of a service with which we tried Shenandoah in OpenJDK 11.08. Thanks to your patch we see parallel cleaning nicely broken out ("CU:" below).
[2020-08-05T17:53:00.395+0000] Concurrent Reset 18790 us
[2020-08-05T17:53:00.395+0000] Pause Init Mark (G) 3899 us
[2020-08-05T17:53:00.395+0000] Pause Init Mark (N) 1455 us
[2020-08-05T17:53:00.395+0000] Accumulate Stats 140 us
[2020-08-05T17:53:00.395+0000] Make Parsable 359 us
[2020-08-05T17:53:00.395+0000] Update Region States 83 us
[2020-08-05T17:53:00.395+0000] Scan Roots 709 us, parallelism: 9.12x
[2020-08-05T17:53:00.395+0000] S: <total> 6467 us
[2020-08-05T17:53:00.395+0000] S: Thread Roots 5668 us, workers (us): 368, 372, 372, 371, 207, 366, 369, 365, 420, 416, 415, 408, 404, 401, 392, 23,
[2020-08-05T17:53:00.395+0000] S: Universe Roots 2 us, workers (us): 2, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] S: JNI Handles Roots 3 us, workers (us): 3, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] S: Synchronizer Roots 0 us, workers (us): 0, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] S: Management Roots 2 us, workers (us): 2, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] S: System Dict Roots 16 us, workers (us): ---, 16, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] S: CLDG Roots 774 us, workers (us): 86, 76, 88, 81, 236, 72, 68, 66, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] S: JVMTI Roots 1 us, workers (us): 1, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] Resize TLABs 53 us
[2020-08-05T17:53:00.395+0000] Concurrent Marking 188933 us
[2020-08-05T17:53:00.395+0000] Pause Final Mark (G) 16049 us
[2020-08-05T17:53:00.395+0000] Pause Final Mark (N) 13660 us
[2020-08-05T17:53:00.395+0000] Finish Queues 647 us
[2020-08-05T17:53:00.395+0000] System Purge 126 us
[2020-08-05T17:53:00.395+0000] Unload Classes 589 us
[2020-08-05T17:53:00.395+0000] Cleanup 10206 us, parallelism: 15.79x
[2020-08-05T17:53:00.395+0000] CU: <total> 161151 us
[2020-08-05T17:53:00.395+0000] CU: Code Cache Roots 56381 us, workers (us): 3588, 3524, 3588, 3537, 3524, 3529, 3539, 3541, 3550, 3502, 3518, 3487, 3483, 3493, 3476, 3502,
[2020-08-05T17:53:00.395+0000] CU: Code Cache Roots(2nd Pass) 10696 us, workers (us): 665, 672, 672, 667, 664, 684, 684, 681, 671, 684, 683, 682, 558, 671, 676, 682,
[2020-08-05T17:53:00.395+0000] CU: String Table Roots 26639 us, workers (us): 1642, 1692, 1627, 1678, 1692, 1663, 1649, 1647, 1646, 1676, 1660, 1681, 1672, 1677, 1688, 1650,
[2020-08-05T17:53:00.395+0000] CU: Resolved Table Roots 134 us, workers (us): 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 133, 0, 0, 0,
[2020-08-05T17:53:00.395+0000] CU: CLDG Roots 67301 us, workers (us): 4199, 4197, 4244, 4223, 4202, 4205, 4198, 4197, 4200, 4199, 4198, 4207, 4236, 4198, 4197, 4201,
[2020-08-05T17:53:00.395+0000] Weak Roots 125 us
[2020-08-05T17:53:00.395+0000] CLDG 1 us
[2020-08-05T17:53:00.395+0000] Update Region States 77 us
[2020-08-05T17:53:00.395+0000] Retire TLABs 353 us
[2020-08-05T17:53:00.395+0000] Choose Collection Set 274 us
[2020-08-05T17:53:00.395+0000] Rebuild Free Set 16 us
[2020-08-05T17:53:00.395+0000] Initial Evacuation 1307 us, parallelism: 9.74x
[2020-08-05T17:53:00.395+0000] E: <total> 12729 us
[2020-08-05T17:53:00.395+0000] E: Thread Roots 7530 us, workers (us): 441, 460, 503, 457, 467, 472, 522, 474, 528, 457, 434, 524, 456, 365, 555, 416,
[2020-08-05T17:53:00.395+0000] E: Code Cache Roots 3356 us, workers (us): 354, 333, 336, 160, 327, 134, 141, 129, 337, 336, 361, 111, 148, 0, 148, 0,
[2020-08-05T17:53:00.395+0000] E: Universe Roots 1 us, workers (us): 1, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] E: JNI Handles Roots 1 us, workers (us): 1, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] E: Synchronizer Roots 0 us, workers (us): 0, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] E: Management Roots 2 us, workers (us): 2, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] E: System Dict Roots 11 us, workers (us): ---, 11, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] E: CLDG Roots 1828 us, workers (us): ---, ---, ---, 190, ---, 190, 190, 190, ---, ---, ---, 254, 191, 431, 191, ---,
[2020-08-05T17:53:00.395+0000] E: JVMTI Roots 1 us, workers (us): 1, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---, ---,
[2020-08-05T17:53:00.395+0000] Concurrent Cleanup 39 us
Now we see that the long poles for STW pauses in this service are the code cache and CLDG root tasks in parallel cleaning ("Cleanup"). If we could subdue those we could consistently get into single-digit ms max pause territory. In later OpenJDK releases, we see efforts to make such phases concurrent, so there is potential for backports. Alternatively, in case there are any problems in any of those cases, one could possibly split up the sub tasks across multiple GC cycles. Do they really all have to run in the same cycle? Are there known dependencies that link some tasks into the same cycle? (I suspect such a link between the class unloading block and the two code cache scanning blocks.)
Bernd
On 8/4/20, 12:20 PM, "Mathiske, Bernd" <mathiske at amazon.com> wrote:
Zhengyu,
It is wonderful to have that patch. Thank you!
I'll apply it and get some numbers.
Bernd
On 8/4/20, 12:09 PM, "Zhengyu Gu" <zgu at redhat.com> wrote:
CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
Forgot to mention, the patch is based on jdk11u.
Thanks,
-Zhengyu
On 8/4/20 3:02 PM, Zhengyu Gu wrote:
> Hi Bern,
>
> Please find my current patch for adding detail timings for class
> unloading cleanup works.
>
> I did not expect string table patch can make meaning impact, because it
> does not seem that many strings die from early logs.
>
> -Zhengyu
>
>
> On 8/4/20 12:52 PM, Mathiske, Bernd wrote:
>> I should mention that we are using "OpenJDK 11.0.8 with Shenandoah" in
>> this context and that's why only half the patch applied. In OpenJDK
>> 15, both parts apply.
>>
>> On 8/4/20, 9:48 AM, "shenandoah-dev on behalf of Mathiske, Bernd"
>> <shenandoah-dev-retn at openjdk.java.net on behalf of
>> mathiske at amazon.com> wrote:
>>
>> That did not help in this case. But thanks for the bug fix anyway!
>> BTW, I could only apply the string table part of the patch, the
>> ResolvedMethod part does not apply.
>>
>> So we've still got that problem with long pauses in final mark.
>> Doubling parallel GC threads brought them from ~20ms to ~13. This only
>> happens in final mark with class unloading. I found that there are
>> three code blocks that could be responsible and maybe I have been
>> looking at the wrong one.
>>
>> void ShenandoahHeap::stw_unload_classes(bool full_gc) {
>> if (!unload_classes()) return;
>> bool purged_class;
>>
>> // BLOCK 1
>> // Unload classes and purge SystemDictionary.
>> {
>> ShenandoahGCPhase phase(full_gc ?
>>
>> ShenandoahPhaseTimings::full_gc_purge_class_unload :
>>
>> ShenandoahPhaseTimings::purge_class_unload);
>> purged_class = SystemDictionary::do_unloading(gc_timer());
>> }
>>
>> // BLOCK 2
>> {
>> ShenandoahGCPhase phase(full_gc ?
>>
>> ShenandoahPhaseTimings::full_gc_purge_par :
>> ShenandoahPhaseTimings::purge_par);
>> ShenandoahIsAliveSelector is_alive;
>> uint num_workers = _workers->active_workers();
>> ShenandoahParallelCleaningTask
>> unlink_task(is_alive.is_alive_closure(), true, true, num_workers,
>> purged_class);
>> _workers->run_task(&unlink_task);
>> }
>>
>> //BLOCK 3
>> {
>> ShenandoahGCPhase phase(full_gc ?
>>
>> ShenandoahPhaseTimings::full_gc_purge_cldg :
>> ShenandoahPhaseTimings::purge_cldg);
>> ClassLoaderDataGraph::purge();
>> }
>> // Resize and verify metaspace
>> MetaspaceGC::compute_new_size();
>> MetaspaceUtils::verify_metrics();
>> }
>>
>> And here are the timings we got in the GC log:
>> ...
>> System Purge 301 us
>> Unload Classes 17824 us
>> Weak Roots 301 us
>> CLDG 1 us
>> ...
>>
>> I suspect "Unload Classes" reports on BLOCK 1 only and not all
>> three then? That would explain a lot. I can't find any reference to
>> "Weak Roots" and "CLDG" seems to reflect BLOCK3, correct?
>>
>> Bernd
>>
>> On 7/30/20, 11:24 AM, "shenandoah-dev on behalf of Mathiske,
>> Bernd" <shenandoah-dev-retn at openjdk.java.net on behalf of
>> mathiske at amazon.com> wrote:
>>
>> Thanks! I'll add the patch and we will report back.
>>
>> On 7/30/20, 11:17 AM, "Zhengyu Gu" <zgu at redhat.com> wrote:
>>
>> CAUTION: This email originated from outside of the
>> organization. Do not click links or open attachments unless you can
>> confirm the sender and know the content is safe.
>>
>>
>>
>> Hi Bernd,
>>
>> On 7/30/20 2:12 PM, Mathiske, Bernd wrote:
>> > Zhengyu,
>> >
>> > is this bug you filed directly related?
>> > https://bugs.openjdk.java.net/browse/JDK-8250841
>> >
>> > Do you expect that the patch you also posted might help
>> here?
>> > (http://cr.openjdk.java.net/~zgu/JDK-8250841/webrev.00/)
>>
>> Yes, it is directly related and should help the pause
>> times, but not
>> sure by how much.
>>
>> -Zhengyu
>>
>> >
>> > Bernd
>> >
>> > On 7/29/20, 1:56 PM, "Zhengyu Gu" <zgu at redhat.com> wrote:
>> >
>> > CAUTION: This email originated from outside of the
>> organization. Do not click links or open attachments unless you can
>> confirm the sender and know the content is safe.
>> >
>> >
>> >
>> > Hi Niall,
>> > >
>> > > Based on all of this, we had some followup
>> questions:
>> > >
>> > > * Is our understanding of
>> -XX:+ClassUnloadingWithConcurrentMark correct - is its only impact to
>> disable class unloading if set to false?
>> > Yes, setting ClassUnloadingWithConcurrentMark to
>> false, still disables
>> > class unloading.
>> >
>> > > * Is there a way to get a detailed breakdown of
>> final mark timing and/or class unloading to expose string/symbol table
>> cleanup time?
>> > Not now. We can add these timings.
>> >
>> > > * Is 16ms a reasonable duration to cleanup
>> string/symbol tables given ~80K strings and ~310K symbols?
>> >
>> > I don't know. Roman & Aleksey may have some insights.
>> >
>> > > * Apart from increasing
>> ShenandoahUnloadClassesFrequency, are there any options to reduce the
>> impact of string/symbol table cleanup on final marking?
>> >
>> > There is possibility that we can move string table
>> cleaning to
>> > concurrent phase. I believe string table is a
>> concurrent hash table in
>> > 11u already.
>> >
>> > Thanks,
>> >
>> > -Zhengyu
>> >
>> >
>> > >
>> > >
>> > > Thanks very much for your help!
>> > >
>> > > Niall and Bernd
>> > >
>> > > On 7/17/20, 08:38, "Zhengyu Gu" <zgu at redhat.com>
>> wrote:
>> > >
>> > > CAUTION: This email originated from outside
>> of the organization. Do not click links or open attachments unless you
>> can confirm the sender and know the content is safe.
>> > >
>> > >
>> > >
>> > > On 7/17/20 11:13 AM, Connaughton, Niall wrote:
>> > > > Thanks Zhengyu, I'm looking into this,
>> I'm seeing a lot of class loaders containing a single class. I'll test
>> with class unloading enabled. Considering it's disabled by default, is
>> there a downside to enabling it?
>> > >
>> > > class unloading is also performed during
>> final mark, so it impacts final
>> > > mark pause as well, but probably without
>> this accumulation effect.
>> > >
>> > > We have a parameter,
>> ShenandoahUnloadClassesFrequency, to control
>> > > frequency of class unloading when it is
>> enabled, default is once every 5
>> > > GC cycles. You may want to tune the
>> parameter to achieve some sort of
>> > > balance.
>> > >
>> > > Thanks,
>> > >
>> > > -Zhengyu
>> > >
>> > >
>> > >
>> > > >
>> > > > Roman & Aleksey, thanks for the details.
>> I'm using 11.0.7, will setup a new test with an 11.0.8 build.
>> > > >
>> > > > Thanks,
>> > > > Niall
>> > > >
>> > > > On 7/17/20, 06:35, "Zhengyu Gu"
>> <zgu at redhat.com> wrote:
>> > > >
>> > > > CAUTION: This email originated from
>> outside of the organization. Do not click links or open attachments
>> unless you can confirm the sender and know the content is safe.
>> > > >
>> > > >
>> > > >
>> > > > Hi Niall,
>> > > >
>> > > > By default, class unloading is
>> disabled for Shenandoah in JDK11.
>> > > > Accumulated class loaders may
>> prolong final mark. You may want to try:
>> > > >
>> > > > jcmd <pid> VM.classloader_stats
>> > > >
>> > > > If it is the case, you may want to
>> try -XX:+ClassUnloadingWithConcurrentMark
>> > > >
>> > > > Thanks,
>> > > >
>> > > > -Zhengyu
>> > > >
>> > > >
>> > > >
>> > > > On 7/17/20 12:56 AM, Connaughton,
>> Niall wrote:
>> > > > > Hey all, firstly thanks for all
>> the work on Shenandoah, I’m excited for its potential for us.
>> > > > >
>> > > > > I’ve been doing some testing and
>> have run across a couple of issues that have me scratching my head.
>> The first is Final Mark pauses are increasing steadily over several
>> days. On day 1, Final Mark pauses are 6.5-8.5ms. By day 3, they’re
>> 12-16ms. The heap occupancy is not ramping up, and neither are the
>> concurrent cycle times, so I’m not sure what’s behind this. The
>> application is running a 20GB heap, peaking around 35% live data heap
>> occupancy, and allocating ~1.3-1.5GB/s.
>> > > > >
>> > > > > What angles can I look at to dig
>> into the cause of increasing Final Mark pauses? I don’t see a lot of
>> details on the Final Mark in the gc logs, and there doesn’t seem to be
>> much difference in the logs over time, except for the pause duration
>> increasing. Here’s an example of a Final Mark log for before/after
>> comparison:
>> > > > >
>> > > > > [2020-07-13T22:27:28.835+0000]
>> GC(2224) Pause Final Mark
>> > > > > [2020-07-13T22:27:28.835+0000]
>> GC(2224) Using 8 of 8 workers for final marking
>> > > > > [2020-07-13T22:27:28.839+0000]
>> GC(2224) Adaptive CSet Selection. Target Free: 2047M, Actual Free:
>> 2429M, Max CSet: 853M, Min Garbage: 0B
>> > > > > [2020-07-13T22:27:28.840+0000]
>> GC(2224) Collectable Garbage: 12171M (88% of total), 176M CSet, 1548
>> CSet regions
>> > > > > [2020-07-13T22:27:28.840+0000]
>> GC(2224) Immediate Garbage: 46151K (0% of total), 11 regions
>> > > > > [2020-07-13T22:27:28.843+0000]
>> GC(2224) Pause Final Mark 7.373ms
>> > > > >
>> > > > > [2020-07-15T23:25:05.780+0000]
>> GC(24251) Pause Final Mark
>> > > > > [2020-07-15T23:25:05.780+0000]
>> GC(24251) Using 8 of 8 workers for final marking
>> > > > > [2020-07-15T23:25:05.787+0000]
>> GC(24251) Adaptive CSet Selection. Target Free: 2047M, Actual Free:
>> 2513M, Max CSet: 853M, Min Garbage: 0B
>> > > > > [2020-07-15T23:25:05.787+0000]
>> GC(24251) Collectable Garbage: 12062M (88% of total), 184M CSet, 1535
>> CSet regions
>> > > > > [2020-07-15T23:25:05.787+0000]
>> GC(24251) Immediate Garbage: 34711K (0% of total), 5 regions
>> > > > > [2020-07-15T23:25:05.792+0000]
>> GC(24251) Pause Final Mark 11.790ms
>> > > > >
>> > > > >
>> > > > > The second issue I ran into was
>> that the pacer seemed to be adding a lot of latency. I couldn’t find
>> any traces in the logs of the pacer’s activity. The summary at
>> shutdown from gc+stats is useful, but having some signs of the pacer
>> in the gc logs as the application runs would help correlate against
>> other logs showing latency spikes. Is there any way to get more
>> visibility on the pacer? Disabling the pacer removed the latency
>> impact and we started seeing some really positive signs on the
>> latency. I was expecting that we’d just see more degenerated GC and
>> the latency would be similar, but this wasn’t the case.
>> > > > >
>> > > > > I’m generally happy running with
>> the pacer disabled, especially as it seems we’ll have more visibility
>> into degenerated GCs than we will over the pacer, so we can track
>> regression more easily. So I’m asking this more for understanding than
>> solving a blocking issue.
>> > > > >
>> > > > > Happy to take any pointers or
>> provide any more info that would help.
>> > > > >
>> > > > > Thanks,
>> > > > > Niall
>> > > > >
>> > > >
>> > > >
>> > >
>> > >
>> >
>> >
>>
>>
>>
>>
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