Hi!
On 23/11/2018 14:55, Jaikiran Pai wrote:
Hi,
I'm looking for some inputs in debugging a high memory usage issue (and
subsequently the process being killed) in one of the applications I deal
with. Given that from what I have looked into this issue so far, this
appears to be something to do with the CMS collector, so I hope this is
the right place to this question.
A bit of a background - The application that I'm dealing with is
ElasticSearch server version 1.7.5. We use Java 8:
java version "1.8.0_172"
Java(TM) SE Runtime Environment (build 1.8.0_172-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.172-b11, mixed mode)
To add to the complexity in debugging this issue, this runs as a docker
container on docker version 18.03.0-ce on a CentOS 7 host VM kernel
version 3.10.0-693.5.2.el7.x86_64.
We have been noticing that this container/process keeps getting killed
by the oom-killer every few days. The dmesg logs suggest that the
process has hit the "limits" set on the docker cgroups level. After
debugging this over past day or so, I've reached a point where I can't
make much sense of the data I'm looking at. The JVM process is started
using the following params (of relevance):
java -Xms2G -Xmx6G -XX:+UseParNewGC -XX:+UseConcMarkSweepGC
-XX:CMSInitiatingOccupancyFraction=75 -XX:+UseCMSInitiatingOccupancyOnly
-XX:+HeapDumpOnOutOfMemoryError -XX:+DisableExplicitGC ....
What is your limit? You will need _more_ than 6 gig of memory; exactly
how much is hard to say. You will probably find the limit faster if you
use Xms==Xmx and maybe not need days of running the application.
As you can see it uses CMS collector with 75% of tenured/old gen for
initiating the GC.
After a few hours/days of running I notice that even though the CMS
collector does run almost every hour or so, there are huge number of
objects _with no GC roots_ that never get collected. These objects
internally seem to hold on to ByteBuffer(s) which (from what I see) as a
result never get released and the non-heap memory keeps building up,
till the process gets killed. To give an example, here's the jmap -histo
output (only relevant parts):
1: 861642 196271400 [B
2: 198776 28623744
org.apache.lucene.codecs.blocktree.SegmentTermsEnumFrame
3: 676722 21655104
org.apache.lucene.store.ByteArrayDataInput
4: 202398 19430208
org.apache.lucene.codecs.lucene41.Lucene41PostingsWriter$IntBlockTermState
5: 261819 18850968 org.apache.lucene.util.fst.FST$Arc
6: 178661 17018376 [C
7: 31452 16856024 [I
8: 203911 8049352 [J
9: 85700 5484800 java.nio.DirectByteBufferR
10: 168935 5405920
java.util.concurrent.ConcurrentHashMap$Node
11: 89948 5105328 [Ljava.lang.Object;
12: 148514 4752448
org.apache.lucene.util.WeakIdentityMap$IdentityWeakReference
....
Total 5061244 418712248
This above output is without the "live" option. Running jmap -histo:live
returns something like (again only relevant parts):
13: 31753 1016096
org.apache.lucene.util.WeakIdentityMap$IdentityWeakReference
...
44: 887 127728
org.apache.lucene.codecs.blocktree.SegmentTermsEnumFrame
...
50: 3054 97728
org.apache.lucene.store.ByteArrayDataInput
...
59: 888 85248
org.apache.lucene.codecs.lucene41.Lucene41PostingsWriter$IntBlockTermState
Total 1177783 138938920
Notice the vast difference between the live and non-live instances of
the same class. This isn't just in one "snapshot". I have been
monitoring this for more than a day and this pattern continues. Even
taking heap dumps and using tools like visualvm shows that these
instances have "no GC root" and I have even checked the gc log files to
see that the CMS collector does occasionally run. However these objects
never seem to get collected.
What makes you believe they never get collected?
I realize this data may not be enough to narrow down the issue, but what
I am looking for is some kind of help/input/hints/suggestions on what I
should be trying to figure out why these instances aren't GCed. Is this
something that's expected in certain situations?
Being killed by the oom-killer suggests that your limit is too low
and/or your -Xmx is too large. If an increasing number of objects does
not get collected, you would get an exception (not being killed by the
oom-killer). What likely is happening is that your java heap slowly
grows (not unusual with CMS that does not do compaction of old objects)
and that the memory consumed by your docker image exceeds your limit.
How big the limit should be is hard to tell, but it _must_ be larger
than your "-Xmx" (the JVM is using more memory than the java heap, so
this would be true even without the addition of docker).
Thanks,
Leo
-Jaikiran
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