At Amazon EMR, we constantly listen to our customers’ challenges with running large-scale Amazon EMR HBase deployments. One consistent pain point that kept emerging is unpredictable application behavior due to garbage collection (GC) pauses on HBase. Customers running critical workloads on HBase were experiencing occasional latency spikes due to varying GC pauses, particularly impacting when they occurred during peak business hours.
To reduce this unpredictable impact to business-critical applications running on HBase, we turn to Oracle’s Z Garbage Collector (ZGC), specifically it’s generational support introduced in JDK 21. Generational ZGC delivers consistent sub-millisecond pause times that dramatically reduce tail latency.
In this post, we examine how unpredictable GC pauses affect business-critical workloads, benefits of enabling generational ZGC in HBase. We also cover additional GC tuning techniques to improve the application throughput and reduce tail latency. Amazon EMR 7.10.0 introduces new configuration parameters that allow you to seamlessly configure and tune the garbage collector for HBase RegionServers.
By incorporating generational collection into ZGC’s ultra-low pause architecture, it efficiently handles both short-lived and long-lived objects, making it exceptionally well-suited to HBase’s workload characteristics:
- Handling mixed object lifetimes – HBase operations create a mix of short-lived objects (such as temporary buffers for read/write operations) and long-lived objects (such as cached data blocks and metadata). Generational ZGC can efficiently manage both, reducing overall GC frequency and impact.
- Adapting to workload patterns – As workload patterns change throughout the day — for instance, from write-heavy ingestion to read-heavy analytics — generational ZGC adapts its collection strategy, maintaining optimal performance.
- Scaling with heap size – As data volumes grow and HBase clusters require larger heaps, generational ZGC maintains it’s sub-millisecond pause times, providing consistent performance even as you scale up.
Understanding the impact of GC pauses on HBase
When running HBase RegionServers, the JVM heap can accumulate a large number of objects, both short-lived (temporary objects created during operations) and long-lived (cached data, metadata). Traditional garbage collectors like Garbage-First Garbage Collector (G1 GC) need to pause application threads during certain phases of garbage collection, particularly during “stop-the-world” (STW) events. GC pauses can have several impacts on HBase :
- Latency spikes – GC pauses introduce latency spikes, often impacting tail latencies (p99.9 and p99.99) of the application which can lead to timeout for client requests and inconsistent response times..
- Application availability – All application threads are halted during STW events and it negatively impacts overall application availability.
- RegionServer failures – If GC pauses exceed the configured ZooKeeper session timeout, they might lead to RegionServer failures.
HBase RegionServer reports whenever there is an unusually long GC pause time using the JvmPauseMonitor. The following log entry shows an example of GC pauses reported by HBase RegionServer. During YCSB benchmarking, G1 GC exhibited 75 such pauses over a 7-hour period, whereas generational ZGC showed no long pauses under identical workload and testing conditions.
G1 GC pauses are proportional to the pressure on the heap and the object allocation patterns. As a result, the pauses might get worse if the heap is under too much load, whereas generational ZGC maintains it’s pause times goals even under high pressure.
Pause time and availability (uptime) comparison: Generational ZGC vs. G1GC in Amazon EMR HBase
Our testing revealed significant differences in GC pause time between the generational ZGC and G1 GC for HBase on Amazon EMR 7.10. We used 1 m5.4xlarge (primary), 5 m5.4xlarge (core) nodes cluster settings and ran multiple iterations of 1-billion rows YCSB workloads to compare the GC pauses and uptime percentage. Based on our test cluster, we observed a GC pause time improvement from over 1 minute, 24 seconds, to under 1 seconds for over an hour-long execution, improving the application uptime from 98.08% to 99.99%.
We conducted extensive performance testing comparing G1 GC and generational ZGC on HBase clusters running on Amazon EMR, using the default heap settings automatically configured based on Amazon Elastic Compute Cloud (Amazon EC2) instance type. The following image shows the comparison in both GC pause time and uptime percentage at a peak load of 3,00,000 requests per second (data sampled over 1 hour).
The following figures show the breakdown of the 1-hour runtime in 10-minute intervals. The left vertical axis measures the uptime, the right vertical axis measures the GC pause time, and the horizontal axis shows the interval. The generational ZGC maintained consistent uptime and pause time in milliseconds, and G1 GC demonstrated inconsistent and decreased uptime, pause times in seconds.
Tail latency comparison: Generational ZGC vs. G1GC in Amazon EMR HBase
One of the most compelling advantages of generational ZGC over G1 GC is its predictable garbage collection behavior and the impact on application tail latency. G1 GC’s collection triggers are non-deterministic, meaning pause times can vary significantly and occur at unpredictable intervals. These unexpected pauses, though generally manageable, can create latency spikes that particularly affect the slowest percentile of operations. In contrast, generational ZGC maintains consistent, sub-millisecond pause times throughout its operation. This predictability proves crucial for applications requiring stable performance, especially at the highest percentiles of latency (99.9th and 99.99th percentiles). Our YCSB benchmark testing reveals the real-world impact of these different approaches. The following graph illustrates tail latency distribution between G1 GC and generational ZGC over a 2-hour sampling period :
Improvements to BucketCache
BucketCache is an off-heap cache in HBase that is used to cache the frequently accessed data blocks and minimize disk I/O. Bucket cache and heap memory works in conjunction and might increase the contention on the heap depending on the workload. Generational ZGC maintains it’s pause time goals even with a terabyte-sized bucket cache. We benchmarked multiple HBase clusters with varying bucket cache sizes and 32 GB RegionServer heap. The following figures show the peak pause times observed over a 1-hour sampling period, comparing G1 GC and generational ZGC performance.
Enabling this feature and additional fine-tuning parameters
To enable this feature, follow the configurations mentioned in the Performance Considerations. In the following sections, we discuss additional fine-tuning parameters to tailor the configuration for your specific use case.
Fixed JVM heap
Batch processing jobs and short-lived applications benefit from dynamic allocation’s ability to adapt to varying input sizes and processing demands when multiple applications co-exist on the same cluster and run with resource constraints. The memory footprint can expand during peak processing and contract when the workload diminishes. However, for production HBase deployments without any co-existing applications in the same fixed heap allocation offers stable, reliable performance.
Dynamic heap allocation is when the JVM flexibly grows and shrinks its memory usage between minimum (-Xms) and maximum (-Xmx) limits based on application needs, returning unused memory to the operating system. However, this flexibility comes at the cost of performance overhead and memory fragmentation. Dynamic allocation seemed flexible, but it created constant disruptions. The JVM was always negotiating with the operating system for memory, leading to performance overhead and fragmentation. On the other hand, fixed heap allocation pre-allocates a constant amount of memory for the JVM at startup and maintains it throughout runtime, providing better performance by reducing memory negotiation overhead with the operating system. To enable this feature, use the following configuration: :
Enable pre-touch
Applications with large heaps can experience more significant pauses when the JVM needs to allocate and fault in new memory pages. Pre-touch (-XX:+AlwaysPreTouch) instructs the JVM to physically touch and commit all memory pages during heap initialization, rather than waiting until they’re first accessed during runtime. This early commitment reduces the latency of on-demand page faults and memory mappings that occur when pages are first accessed, resulting in more predictable performance especially during heavy load situations. By pre-touching memory pages at startup, you trade a slightly longer JVM startup time for more consistent runtime performance. To enable pre-touch for your HBase cluster, use the following configuration :
Increasing memory mappings for large heaps
Depending on the workload and scale, you might need to increase the Java heap size to accommodate large data in memory. When using the generational ZGC with a large heap setup, it’s critical to also increase the operating system’s memory mapping limit (vm.max_map_count).
When a ZGC-enabled application starts, the JVM proactively checks the system’s vm.max_map_count value. If the limit is too low to support the configured heap, it will issue the following warning :
To increase the memory mappings, use the following configuration and adjust the count value in the command based on the heap size of the application.
Conclusion
The introduction of generational ZGC and fixed heap allocation for HBase on Amazon EMR marks a significant leap forward in the predictable performance and tail latency reduction. By addressing the long-standing challenges of GC pauses and memory management, these features unlock new levels of efficiency and stability for Amazon EMR HBase deployments. Although the performance improvements vary depending on workload characteristics, you can expect to see significant enhancements in your Amazon EMR HBase clusters’ responsiveness and stability. As data volumes continue to grow and low-latency requirements become increasingly stringent, features like generational ZGC and fixed heap allocation become indispensable. We encourage HBase users on Amazon EMR to enable these features and experience the benefits firsthand. As always, we recommend testing in a staging environment that mirrors your production workload to fully understand the impact and optimize configurations for your specific use case.
Stay tuned for more innovations as we continue to push the boundaries of what’s possible with HBase on Amazon EMR.
About the authors
Vishal Chaudhary is a Software Development Engineer at Amazon EMR. His expertise is in Amazon EMR, HBase and Hive Query Engine. His dedication towards solving distributed system problems is helping Amazon EMR to achieve higher performance improvements.
Ramesh Kandasamy is an Engineering Manager at Amazon EMR. He is a long tenured Amazonian dedicated to solve distributed systems problems.
