Number of CMS collections and the JMM GC counters

Krystal Mok rednaxelafx at gmail.com
Thu Jul 28 16:33:50 UTC 2011


Hi Kevin,

Thank you for taking care of this, and it's good to see the problem is
verified.

I think whether or not the suggested fix is sufficient depends on what paths
can reach CMSCollector::do_compaction_work(). If all paths that can
reach CMSCollector::do_compaction_work() come from
GenCollectedHeap::do_collection(), then the fix should be good to go.
Otherwise it'll need a better workaround.

I believe all concurrent mode failures/interrupts (which includes the
System.gc() case) does come from GenCollectedHeap::do_collection(), but I'm
not exactly sure about this, could anybody please clarify on it?

Regards,
Kris Mok

On Thu, Jul 28, 2011 at 10:12 PM, Kevin Walls <kevin.walls at oracle.com>wrote:

> **
> Hi --
>
> 6580448 was marked as a duplicate of 6581734, which fixed the fact that CMS
> collections were just not counted at all - with CMS, only a stop the world
> full gc would be counted in the stats.
>
> But looks like you're right...  Here is a quick variation of the testcase
> from 6581734 which shows the same thing, and this verifies the same, and is
> solved by ExplicitGCInvokesConcurrent.  If there is no other feedback I can
> test if the removal of the TraceCMSMemoryManagerStats() call in
> CMSCollector::do_compaction_work is all we need...
>
> Regards
> Kevin
>
>
> /*
>  * Copyright (c) 2011, Oracle and/or its affiliates. All rights reserved.
>  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
>  *
>  * This code is free software; you can redistribute it and/or modify it
>  * under the terms of the GNU General Public License version 2 only, as
>  * published by the Free Software Foundation.
>  *
>  * This code is distributed in the hope that it will be useful, but WITHOUT
>  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
>  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
>  * version 2 for more details (a copy is included in the LICENSE file that
>  * accompanied this code).
>  *
>  * You should have received a copy of the GNU General Public License
> version
>  * 2 along with this work; if not, write to the Free Software Foundation,
>  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
>  *
>  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
>  * or visit www.oracle.com if you need additional information or have any
>  * questions.
>  */
>
> /*
>  * @test TestFullGCount.java
>  * @bug
>  * @summary
>  * @run main/othervm -XX:+UseConcMarkSweepGC TestFullGCCount
>  *
>  */
> import java.util.*;
> import java.lang.management.*;
>
>
>
> public class TestFullGCCount {
>
> private String poolName = "CMS";
> private String collectorName = "ConcurrentMarkSweep";
>
> public static void main(String [] args) {
>
>     TestFullGCCount t = null;
>     if (args.length==2) {
>         t = new TestFullGCCount(args[0], args[1]);
>     } else {
>         System.out.println("Defaulting to monitor CMS pool and
> collector.");
>         t = new TestFullGCCount();
>     }
>     t.run();
>
> }
>
> public TestFullGCCount(String pool, String collector) {
>     poolName = pool;
>     collectorName = collector;
> }
> public TestFullGCCount() {
> }
>
> public void run() {
>
>     int count = 0;
>     int iterations = 20;
>     long counts[] = new long[iterations];
>     boolean diffAlways2 = true; // assume we will fail
>
>     for (int i=0; i<iterations; i++) {
>         System.gc();
>         counts[i] = checkStats();
>         if (i>0) {
>             if (counts[i] - counts[i-1] != 2) {
>                 diffAlways2 = false;
>             }
>         }
>     }
>     if (diffAlways2) {
>         throw new RuntimeException("FAILED: difference in count is always
> 2.");
>     }
>     System.out.println("Passed.");
> }
>
> private long checkStats() {
>     long count = 0;
>     List<MemoryPoolMXBean> pools =
> ManagementFactory.getMemoryPoolMXBeans();
>     List<GarbageCollectorMXBean> collectors =
> ManagementFactory.getGarbageCollectorMXBeans();
>     for (int i=0; i<collectors.size(); i++) {
>         GarbageCollectorMXBean collector = collectors.get(i);
>         String name = collector.getName();
>         if (name.contains(collectorName)) {
>             System.out.println(name + ": collection count = "
>                                + collector.getCollectionCount());
>             count = collector.getCollectionCount();
>         }
>     }
>     return count;
>
> }
>
> }
>
>
> On 27/07/11 17:12, Krystal Mok wrote:
>
> Hi all,
>
>  I've been looking at a strange inconsistency of full GC count recorded by
> jvmstat and JMM counters. I'd like to know which ones of the following
> behaviors are by design, which ones are bugs, and which ones are just my
> misunderstanding. I apologize for making a short story long...
>
>  =====================================================
>
>  The counters involved:
>
>  * A jvmstat counter named "sun.gc.collector.1.invocations" keeps track of
> the number of pauses occured as a result of a major collection. It is used
> by utilities such as jstat as the source of "FGC" (full collection count),
> and the old gen collection count in Visual GC. It's updated by an
> TraceCollectorStats object.
> * A JMM counter, GCMemoryManager::_num_collections, keeps track of the
> number of collections that have ended. This counter is used as HotSpot's
> implementation of the JMX GarbageCollectorMXBean.getCollectionCount(). It's
> updated by either a TraceMemoryManagerStats object or a
> TraceCMSMemoryManagerStats object.
>
>  To show the situation, I've made a screenshot of a VisualVM and a
> JConsole running side by side, both are monitoring the VisualVM's GC stats:
>
> http://dl.iteye.com/upload/attachment/524811/913cb0e1-7add-3ac0-a718-24ca705cad22.png
> (I'll upload the screenshot to somewhere else if anybody can't see it)
> The VisualVM instance is running on JDK6u26, with ParNew+CMS.
> In the screenshot, Visual GC reports that the old gen collection count is
> 20, while JConsole reports 10.
>
>  I see that there was this bug:
> 6580448: CMS: Full GC collection count mismatch between
> GarbageCollectorMXBean and jvmstat (VisualGC)
> I don't think the current implementation has a bug in the sense that the
> two counters don't report the same number.
>
>  This behavior seems reasonable, but the naming of the value in these
> tools are confusing: both tools say "collections", but apparently the number
> in Visual GC means "number of pauses" where as the number in JConsole means
> "number of collection cycles". It'd be great if the difference could be
> documented somewhere, if that's the intended behavior.
>
>  And then the buggy behavior. Code demo posted on gist:
> https://gist.github.com/1106263
> Starting from JDK6u23, when using CMS without ExplicitGCInvokesConcurrent,
> System.gc() (or Runtime.getRuntime().gc(), or MemoryMXBean.gc() via JMX)
> would make the JMM GC counter increment by 2 per invocation, while the
> jvmstat counter is only incremented by 1. I believe the latter is correct
> and the former needs some fixing.
>
>  =====================================================
>
>  My understanding of the behavior shown above:
>
>  1. The concurrent GC part:
>
>  There are 2 pauses in a CMS concurrent GC cycle, one in the initial mark
> phase, and one in the final remark phase.
> To trigger a concurrent GC cycle, the CMS thread wakes up periodically to
> see if it shouldConcurrentCollect(), and trigger a cycle when the predicate
> returned true, or goes back to sleep if the predicate returned false. The
> whole concurrent GC cycle doesn't go through
> GenCollectedHeap::do_collection().
>
>  The jvmstat counter for old gen pauses is updated in
> CMSCollector::do_CMS_operation(CMS_op_type op), which exactly covers both
> pause phases.
>
>  The JMM counter, however, is updated in the concurrent sweep phase,
> CMSCollector::sweep(bool asynch), if there was no concurrent mode failure;
> or it is updated in CMSCollector::do_compaction_work(bool
> clear_all_soft_refs) in case of a bailout due to concurrent mode failure
> (advertised as so in the code comments). So that's an increment by 1 per
> concurrent GC cycle, which does reflect the "number of collection cycles",
> fair enough.
>
>  So far so good.
>
>  2. The System.gc() part:
>
>  Without ExplicitGCInvokesConcurrent set, System.gc() does a
> stop-the-world full GC, which consists of only one pause, so "number of
> pauses" would equal "number of collections" in this case.
> It will go into GenCollectedHeap::do_collection(); both the jvmstat and the
> JMM GC counter gets incremented by 1 here,
>
>  TraceCollectorStats tcs(_gens[i]->counters());
> TraceMemoryManagerStats tmms(_gens[i]->kind());
>
>  But, drilling down into:
> _gens[i]->collect(full, do_clear_all_soft_refs, size, is_tlab);
>
>  That'll eventually go into:
> CMSCollector::acquire_control_and_collect(bool full, bool
> clear_all_soft_refs)
>
>  System.gc() is user requested so that'll go further into
> mark-sweep-compact:
> CMSCollector::do_compaction_work(bool clear_all_soft_refs)
> And here, it increments the JMM GC counter again (remember it was in the
> concurrent GC path too, to handle bailouts), even though this is still in
> the same collection. This leads to the "buggy behavior" mentioned earlier.
>
>  The JMM GC counter wasn't added to CMS until this fix got in:
> 6581734: CMS Old Gen's collection usage is zero after GC which is incorrect
>
>  The code added to CMSCollector::do_compaction_work() works fine in the
> concurrent GC path, but interacts badly with the original logic in
> GenCollectedHeap::do_collection().
>
>  =====================================================
>
>  I thought all concurrent mode failures/interrupts come from
> GenCollectedHeap::do_collection(). If that's the case, then it seems
> unnecessary to update the JMM GC counter in
> CMSCollector::do_compaction_work(), simply removing it should fix the
> problem.
>
>  With that, I'd purpose the following (XS) change: (diff against HS20)
>
>  diff -r f0f676c5a2c6
> src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp
> ---
> a/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp
>      Tue Mar 15 19:30:16 2011 -0700
> +++
> b/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp
>      Thu Jul 28 00:02:41 2011 +0800
> @@ -2022,9 +2022,6 @@
>
>  _intra_sweep_estimate.padded_average());
>    }
>
>  -  {
> -    TraceCMSMemoryManagerStats();
> -  }
>     GenMarkSweep::invoke_at_safepoint(_cmsGen->level(),
>      ref_processor(), clear_all_soft_refs);
>    #ifdef ASSERT
>
>  The same goes for the changes in:
> 7036199: Adding a notification to the implementation of
> GarbageCollectorMXBeans
>
>  =====================================================
>
>  P.S. Is there an "official" name for the counters that I referred to as
> "jvmstat counters" above? Is it just jvmstat, or PerfData or HSPERFDATA?
>
>
>
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