Concerns Related to GC
Based on an extensive testing campaign we performed in June-August 2020, we extracted some points of advice on how to choose the right JDK/GC combination and how to tune your setup to the workload of your Hazelcast Jet data pipeline.
Upgrade Your JDK
If you are still on JDK 8, seriously consider upgrading. We found that none of its garbage collectors are a match for the offerings of JDK 11, which is the current version with Oracle's Long-Term Support (LTS). The JVM has been undergoing a phase of rapid development lately, which means you can expect numerous improvements with each JDK version.
The G1 Collector is Great for Most Workloads
For batch workloads, as well as streaming workloads that can tolerate
occasional latency spikes of 2-3 seconds, the G1 collector is the best
choice because it has very good throughput and its failure modes are
graceful. It performs very well in a variety of workloads without any
tuning parameters. Its default target for the maximum stop-the-world GC
pause is 200 ms and you can configure it lower, down to 5 ms (using
-XX:MaxGCPauseMillis). Lower targets allow less throughput, though.
The mentioned 2-3 seconds latency (as opposed to the usual 0.2 seconds)
occurs only in exceptional conditions with very high GC pressure. The
advantage of G1 over many other collectors is a graceful increase in GC
pause length under such conditions.
For Latency Goals Below 10 ms, Consider a Low-Latency GC
If you aim for very low latencies (anything below 10 ms), you can
achieve it with G1 as well, but you will probably have to use the
-XX:MaxNewSize flag in order to constrain the Minor GC pause duration.
In our test we found the values
200m to work best over our range of throughputs, lower values
being better for lower throughputs.
If your data pipeline doesn't have too large state (i.e., less than a
million keys within a sliding window), you can consider the Z garbage
collector. We found it to work well without any tuning parameters. Its
current downside is that it handles less throughput compared to G1 and,
being non-generational, doesn't work well if you have a lot of static
data on the heap (for example, if your data pipeline has a
stage). ZGC is an experimental collector under intense development, so
you can expect further improvements, including generational GC behavior,
in the future.
In our tests we found that as of JDK version 14.0.2, the other low-latency collector, Shenandoah, still did not perform as well as ZGC and latencies with it exceeded 10 ms in many cases.
Reduce the Jet Cooperative Thread Pool
A concurrent garbage collector uses a number of threads to do its work
in the background. It uses some static heuristics to determine how many
to use, mostly based on the number of
availableProcessors that the JVM
reports. For example, on a 16-vCPU EC2 c5.4xlarge instance:
- ZGC uses 2 threads
- G1 uses 3 threads
- Shenandoah uses 4 threads
The number of GC threads is configurable through
but we found it best to leave the default setting. However, it is
important to find out the number of GC threads and set Hazelcast Jet's
ConGCThreads). This will allow the GC threads
to be assigned to their own CPU cores, alongside Jet's threads, and thus
the OS can avoid having to interleave Jet and GC threads on the same
A Hazelcast Jet data pipeline may use additional threads for non-cooperative tasklets, in this case you may consider adjusting the cooperative thread pool size even lower.
Egregious Amounts of Free Heap Help Latency
The data pipeline in our tests used less than 1 GB of heap, but we
needed at least
-Xmx=4g to get a good 99.99% latency. We also tested
-Xmx=8g (less than 15% heap usage), and it made the latencies
For Batch Processing, Garbage-Free Aggregation is a Big Deal
In batch aggregation, once a given grouping key is observed, the state associated with it is retained until the end of the computation. If updating that state doesn't create garbage, the whole aggregation process is garbage-free. The computation still produces young garbage, but since most garbage collectors are generational, this has significantly less cost. In our tests, garbage-free aggregation boosted the throughput of the batch pipeline by 30-35%.
For this reason we always strive to make the aggregate operations we
provide with Jet garbage-free. Examples are summing, averaging and
finding extremes. Our current implementation of linear trend, however,
does generate garbage because it uses immutable
BigDecimals in the
If your requirements call for a complex aggregate operation not provided by Jet, and if you use Jet for batch processing, putting extra effort into implementing a custom garbage-free aggregate operation can be worth it.