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From Piotr Nowojski <pi...@data-artisans.com>
Subject Re: Off heap memory issue
Date Wed, 15 Nov 2017 12:18:13 GMT
Hi,

I have been able to observe some off heap memory “issues” by submitting Kafka job provided
by Javier Lopez (in different mailing thread). 

TL;DR;

There was no memory leak, just memory pool “Metaspace” and “Compressed Class Space”
are growing in size over time and are only rarely garbage collected. In my test case they
together were wasting up to ~7GB of memory, while my test case could use as little as ~100MB.
Connect with for example jconsole to your JVM, check their size and cut their size by half
by setting:

env.java.opts: -XX:CompressedClassSpaceSize=***M -XX:MaxMetaspaceSize=***M

In flink-conf.yaml. Everything works fine and memory consumption still too high? Rinse and
repeat.


Long story:

In default settings, with max heap size of 1GB, off heap memory consumption, memory consumption
off non-heap memory pools of “Metaspace” and “Compressed Class Space” was growing
in time which seemed like indefinitely, and Metaspace was always around ~6 times larger compared
to compressed class space. Default max meatspace size is unlimited, while “Compressed class
space” has a default max size of 1GB. 

When I decreased the CompressedClassSpaceSize down to 100MB, memory consumption grew up to
90MB and then it started bouncing up and down by couple of MB. “Metaspace” was following
the same pattern, but using ~600MB. When I decreased down MaxMetaspaceSize to 200MB, memory
consumption of both pools was bouncing around ~220MB.

It seems like there are no general guide lines how to configure those values, since it’s
heavily application dependent. However this seems like the most likely suspect of the apparent
OFF HEAP “memory leak” that was reported couple of times in use cases where users are
submitting hundreds/thousands of jobs to Flink cluster. For more information please check
here:

https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/considerations.html <https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/considerations.html>

Please let us know if this solves your issues.

Thanks, Piotrek

> On 13 Nov 2017, at 16:06, Flavio Pompermaier <pompermaier@okkam.it> wrote:
> 
> Unfortunately the issue I've opened [1] was not a problem of Flink but was just caused
by an ever increasing job plan.
> So no help from that..Let's hope to find out the real source of the problem.
> Maybe using  -Djdk.nio.maxCachedBufferSize could help (but I didn't try it yet)
> 
> Best,
> Flavio
> 
> [1] https://issues.apache.org/jira/browse/FLINK-7845 <https://issues.apache.org/jira/browse/FLINK-7845>
> 
> On Wed, Oct 18, 2017 at 2:07 PM, Kien Truong <duckientruong@gmail.com <mailto:duckientruong@gmail.com>>
wrote:
> Hi,
> 
> We saw a similar issue in one of our job due to ByteBuffer memory leak[1]. 
> We fixed it using the solution in the article, setting -Djdk.nio.maxCachedBufferSize
> 
> This variable is available for Java > 8u102
> 
> Best regards,
> 
> Kien
> [1]http://www.evanjones.ca/java-bytebuffer-leak.html <http://www.evanjones.ca/java-bytebuffer-leak.html>
> 
> On 10/18/2017 4:06 PM, Flavio Pompermaier wrote:
>> We also faced the same problem, but the number of jobs we can run before restarting
the cluster depends on the volume of the data to shuffle around the network. We even had problems
with a single job and in order to avoid OOM issues we had to put some configuration to limit
Netty memory usage, i.e.:
>>  - Add to flink.yaml -> env.java.opts: -Dio.netty.recycler.maxCapacity.default=1
>>  - Edit taskmanager.sh and change TM_MAX_OFFHEAP_SIZE from 8388607T to 5g
>> 
>> At this purpose we wrote a small test to reproduce the problem and we opened an issue
for that [1].
>> We still don't know if the problems are related however..
>> 
>> I hope that could be helpful,
>> Flavio
>> 
>> [1] https://issues.apache.org/jira/browse/FLINK-7845 <https://issues.apache.org/jira/browse/FLINK-7845>
>> 
>> On Wed, Oct 18, 2017 at 10:48 AM, Javier Lopez <javier.lopez@zalando.de <mailto:javier.lopez@zalando.de>>
wrote:
>> Hi Robert,
>> 
>> Sorry to reply this late. We did a lot of tests, trying to identify if the problem
was in our custom sources/sinks. We figured out that none of our custom components is causing
this problem. We came up with a small test, and realized that the Flink nodes run out of non-heap
JVM memory and crash after deployment of thousands of jobs. 
>> 
>> When rapidly deploying thousands or hundreds of thousands of Flink jobs - depending
on job complexity in terms of resource consumption - Flink nodes non-heap JVM memory consumption
grows until there is no more memory left on the machine and the Flink process crashes. Both
TaskManagers and JobManager exhibit the same behavior. The TaskManagers die faster though.
The memory consumption doesn't decrease after stopping the deployment of new jobs, with the
cluster being idle (no running jobs). 
>> 
>> We could replicate the behavior by the rapid deployment of the WordCount Job provided
in the Quickstart with a Python script.  We started 24 instances of the deployment script
to run in parallel.
>> 
>> The non-heap JVM memory consumption grows faster with more complex jobs, i.e. reading
from Kafka 10K events and printing to STDOUT( * ). Thus less deployed jobs are needed until
the TaskManagers/JobManager dies.
>> 
>> We employ Flink 1.3.2 in standalone mode on AWS EC2 t2.large nodes with 4GB RAM inside
Docker containers. For the test, we used 2 TaskManagers and 1 JobManager.
>> 
>> ( * ) a slightly changed Python script was used, which waited after deployment 15
seconds for the 10K events to be read from Kafka, then it canceled the freshly deployed job
via Flink REST API.
>> 
>> If you want we can provide the Scripts and Jobs we used for this test. We have a
workaround for this, which restarts the Flink nodes once a memory threshold is reached. But
this has lowered the availability of our services.
>> 
>> Thanks for your help.
>> 
>> On 30 August 2017 at 10:39, Robert Metzger <rmetzger@apache.org <mailto:rmetzger@apache.org>>
wrote:
>> I just saw that your other email is about the same issue.
>> 
>> Since you've done a heapdump already, did you see any pattern in the allocated objects?
Ideally none of the classes from your user code should stick around when no job is running.
>> What's the size of the heap dump? I'm happy to take a look at it if it's reasonably
small.
>> 
>> On Wed, Aug 30, 2017 at 10:27 AM, Robert Metzger <rmetzger@apache.org <mailto:rmetzger@apache.org>>
wrote:
>> Hi Javier,
>> 
>> I'm not aware of such issues with Flink, but if you could give us some more details
on your setup, I might get some more ideas on what to look for.
>> 
>> are you using the RocksDBStateBackend? (RocksDB is doing some JNI allocations, that
could potentially leak memory)
>> Also, are you passing any special garbage collector options? (Maybe some classes
are not unloaded)
>> Are you using anything else that is special (such as protobuf or avro formats, or
any other big library)?
>> 
>> Regards,
>> Robert
>> 
>> 
>> 
>> On Mon, Aug 28, 2017 at 5:04 PM, Javier Lopez <javier.lopez@zalando.de <mailto:javier.lopez@zalando.de>>
wrote:
>> Hi all,
>> 
>> we are starting a lot of Flink jobs (streaming), and after we have started 200 or
more jobs we see that the non-heap memory in the taskmanagers increases a lot, to the point
of killing the instances. We found out that every time we start a new job, the committed non-heap
memory increases by 5 to 10MB. Is this an expected behavior? Are there ways to prevent this?
>> 
>> 
>> 
>> 
> 
> 
> 
> -- 
> Flavio Pompermaier
> Development Department
> 
> OKKAM S.r.l.
> Tel. +(39) 0461 041809


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