The document outlines Knewton's experience with JVM tuning for Cassandra clusters, highlighting challenges and successful strategies implemented to improve database performance. Key initiatives included updating memtable allocation settings, changing to garbage first garbage collection (G1GC), and enhancing monitoring tools. The findings emphasize the importance of testing configuration changes in collaboration with the development team to optimize performance and reduce technical debt.

1. Java tuning for Knewton’s C* clusters
Lessons learned
Carlos Monroy
Knewton

2. Knewton
Leader in adaptive learning
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previous behavior.
- Advanced content ingestion, curation, and calibration
- Runs in AWS with many different storage backends
- Check us out: www.knewton.com/about/careers/
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3. 1 JVM tuning at Knewton
2 Updating memtable_allocation_type
3 Changing garbage collection strategy
3© DataStax, All Rights Reserved.

4. Context
As many startups, our company needed to make tradeoffs in order to rapidly deliver the product:
- Technical debt.
- Silos and isolated efforts.
- Decisions based on gut and intuition.
One year ago:
- Different versions of Cassandra
- Multiple clients (i.e.: Pycassa, Hector, Astyanax, Datastax)
- Huge challenge with backups and restores
Now:
- 99.98% database uptime
- The database is not a black box anymore
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5. Successful initiatives
- In house command line tools
- cassandra-toolbox python package
- distributed nodetool
- Separation of objects from heap memory (memtable_allocation_type)
- Customization of heap size allocation.
- Update to Garbage First Garbage Collection (G1GC).
- Monitoring/alerting based on JMX metrics
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6. Successful initiatives
- In house command line tools https://github.com/Knewton/cassandra-toolbox/
- cassandra-toolbox python package
- distributed nodetool
- Separation of objects from heap memory (memtable_allocation_type)
- Customization of heap size allocation.
- Update to Garbage First Garbage Collection (G1GC).
- Monitoring/alerting based on JMX metrics
© DataStax, All Rights Reserved. 6

7. Successful initiatives
- In house command line tools
- cassandra-toolbox python package
- distributed nodetool
- Separation of objects from heap memory (memtable_allocation_type)
- Customization of heap size allocation.
- Update to Garbage First Garbage Collection (G1GC).
- Monitoring/alerting based on JMX metrics
© DataStax, All Rights Reserved. 7

8. Successful initiatives
- In house command line tools
- cassandra-toolbox python package
- distributed nodetool
- Separation of objects from heap memory (memtable_allocation_type)
- Customization of heap size allocation. https://tech.knewton.com/
- Update to Garbage First Garbage Collection (G1GC).
- Monitoring/alerting based on JMX metrics
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9. Some less successful initiatives
- Monitoring and alerts based on Graphite graphs
- Too many resources to get an aggregate
- High incidence of false positives and false negatives
- GoCD
- Cloudwatch
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10. 1 JVM tuning at Knewton
2 Updating memtable_allocation_type
3 Changing garbage collection strategy
10© DataStax, All Rights Reserved.

11. memtable_allocation_type
Cassandra allows to keep memtables and key cache objects in the native memory, instead of the Java
JVM heap.
- Used for data structures that continue growing with time
- Options:
- heap_buffers
- default value before Cassandra 3.0
- all the objects are kept in the JVM heap memory
- offheap_buffers
- cell name and values are moved to DirectBuffer objects
- offheap_objects
- moves the entire cell off heap, leaving only a pointer
11

12. Update memtable_allocation_type
cassandra-stress tool is a great starting point while
validating changes for the database configuration
12
But we needed to go the extra mile with an end-to-end test
- involving the rest of the dev team
- demonstrate the positive impact of the change to the
rest of the system

13. Update memtable_allocation_type
cassandra-stress tool is a great starting point while
validating changes for the database configuration
13
But we needed to go the extra mile with an end-to-end test
- involving the rest of the dev team
- demonstrate the positive impact of the change to the
rest of the system

14. Test memtable_allocation_type update
14
Update setting
Load test
(locust)
Compile logs
from C* and
application
Analysis
with R
Response times
Functional load
tests

15. Update memtable - Criteria
End-to-end
15
• Response time
– Timeouts
• Errors
• Throughput
• CPU consumption
• Memory used
Cassandra specific
• Cassandra
– Time spent for Garbage
Collection
• Collection
– Read and Write latencies
– Errors/Exceptions

16. Update memtable_allocation_type
Time used for Garbage Collection
offheap_buffers offheap_objects heap_buffers
16
Comparing garbage collection times with different values for memtable_allocation_type

17. Update memtable_allocation_type
Time used for Garbage Collection
offheap_buffers offheap_objects heap_buffers
17
Comparing garbage collection times with different values for memtable_allocation_type

18. Update memtable_allocation_type
Memory sizes
offheap_buffers offheap_objects heap_buffers
18
Comparing the sizes per generation spaces, before and after the garbage collection.

19. Update memtable_allocation_type
GC phases
offheap_buffers offheap_objects heap_buffers
19
Comparing the behaviour of the garbage collection phases.

20. memtable_allocation_type results
We are using offheap_buffers as it showed:
- the lowest average response time for requests
- lowest CPU usage
- lowest thread count created
- lowest write latency
*Results may vary
20

21. 1 JVM tuning at Knewton
2 Updating memtable_allocation_type
3 Changing garbage collection strategy
21© DataStax, All Rights Reserved.

22. 22
Garbage First Garbage Collection (G1GC)
The G1 collector utilizes multiple background threads to scan through the heap
that it divides into regions.
It is named “Garbage first” (G1) gives preference to scan those regions that
contain the most garbage objects first.
This collector is turned on using the –XX:+UseG1GC flag.

23. G1GC analysis
G1 was released since April 2012 (JDK 7 update 4 and up)
The tools available for the analysis of the garbage collection logs didn’t have
the support or were not able to interpret all the information from our servers.
- Netflix gcviz does not support Garbage First (G1) strategy
- In Oracle’s developer blog (Jeff Taylor) it is proposed an initial approach
for JDK 7
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24. Test garbage collection
24
Enable gc data
collection
Get a
baseline
Compile GC
logs
Analysis
with R

25. G1GC Java arguments
25
Java Arguments as defined in cassandra-env.sh
-XX:+UseG1GC
-XX:G1RSetUpdatingPauseTimePercent=5
-XX:ParallelGCThreads=2
-XX:ConcGCThreads=2
-XX:+PrintGCDetails
-XX:+PrintGCDateStamps
-XX:+PrintHeapAtGC
-XX:+PrintTenuringDistribution
-XX:+PrintGCApplicationStoppedTime
-XX:+PrintPromotionFailure
-XX:PrintFLSStatistics=1
-Xloggc:/<valid path>/gc.log
-XX:+UseGCLogFileRotation
-XX:NumberOfGCLogFiles=10

26. G1GC analysis - Heap size
26

27. G1GC analysis - Heap size
27

28. G1GC analysis - Heap size
28

29. G1GC analysis - Heap size
29

30. G1GC analysis - Heap size
30

31. G1GC analysis - phases
31

32. G1GC Analysis demo

33. Code
Garbage collection analysis :
https://gist.github.com/roymontecutli/4cf5c97f03720e60825f414667c141da
Cassandra toolbox : https://github.com/Knewton/cassandra-toolbox
33

34. Conclusions
- Moving objects from the JVM heap memory can improve
the performance of the application when dealing with
large data sets. Yet you need to find out which strategy
(take out buffers or objects) suits the best for your use
case.
- Garbage Collection is an operation that can impact
adversely the performance on a Cassandra cluster.
Having tools to analyze its behaviour will help to identify
areas of impact and measure improvements.
- Configuration changes should always consider the
system as a whole, involve all the teams.
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35. Thanks
carlos@knewton.com