For most Java server applications without strict pause-time requirements, start with the garbage collector selected by your runtime—G1 in Oracle JDK 25’s server-class configuration—and measure before changing it. Consider ZGC or Shenandoah when application tail latency makes pauses a demonstrated problem, then compare them under the same workload and resource limits. None is universally fastest: pause behavior, throughput, CPU use, and memory headroom depend on the application and JDK build.
How G1, ZGC, and Shenandoah differ
| Collector | Design and objective | Trade-offs and qualifications | When to test it |
|---|---|---|---|
| G1 | Generational, region-based collector that combines concurrent work with stop-the-world evacuation pauses. It adapts collection to pursue pause-time goals while maintaining throughput. | Its pause target is a soft goal, not a maximum guaranteed for every pause. Concurrent work uses CPU, and large or humongous allocations, marking pressure, and evacuation problems can affect behavior. | A sensible starting point for conventional server workloads, particularly when pause needs are not strict. Use logs and measurements to guide tuning. |
| ZGC | Concurrent low-latency collector designed to make pause times independent of heap size. Oracle’s Java SE 25 command reference describes pauses of a few milliseconds, at some throughput cost. | Concurrent collection consumes CPU, and the heap needs room for the live set and allocations that occur while collection runs. Oracle’s documented heap range is 8 MB to 16 TB; that range is not a performance promise. | Test when tail latency is a priority, including with large heaps, and measure throughput, CPU, and allocation headroom. |
| Shenandoah | OpenJDK describes concurrent marking and compaction intended to reduce pauses so they are no longer directly proportional to heap size. Current command documentation distinguishes single-generation satb and generational modes. |
Availability and supported modes depend on the JDK vendor and build. Concurrent work needs CPU and memory headroom; no universal performance winner is established. | Test when low-pause behavior matters and the deployed build supports the collector and desired mode. |
What G1’s pause-time goal actually means
G1 divides the heap into regions, tracks candidate regions, and evacuates live objects from selected regions. Some collection work happens concurrently, while operations such as evacuation take place during pauses. Its adaptive policy tries to meet a pause goal with high probability over time; it does not guarantee a maximum pause duration. Oracle explicitly says G1 is not a real-time collector in its Java SE 25 G1 guide.
Oracle’s Java SE 25 command reference documents 200 ms as the default value of -XX:MaxGCPauseMillis, but that value is a target, not a promise that each pause will finish within 200 ms. Raising or lowering the target can shift the latency/throughput balance; it does not turn G1 into a hard real-time system. See Oracle’s G1 tuning guide before changing settings.
Oracle positions G1 for a broad range of workloads, including heaps in the tens of gigabytes or larger, substantial live sets, variable allocation or promotion, and fragmentation concerns. This is workload guidance, not a minimum heap size or a reason to rule G1 out on smaller heaps. Oracle’s general advice is to start with the VM default unless pause requirements are strict; the choice depends on live data, heap size, and processor availability. See Available Collectors.
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What low-pause collectors can—and cannot—solve
ZGC
Oracle’s Java SE 25 java command reference describes ZGC as a low-latency collector with maximum pause times of a few milliseconds, at some throughput cost, and says pause times are independent of heap size. Treat that as the documented design and behavior description for that release, not as a guarantee of zero pauses, identical performance across workloads, or a fixed end-to-end application response time.
Concurrent collection moves more work out of pauses, but does not make that work free. ZGC still needs CPU to collect and enough heap capacity to accommodate the live set and allocations during a cycle. A workload with high allocation rates or limited CPU can therefore see costs elsewhere even if pauses are short. Oracle documents a supported heap-size range of 8 MB to 16 TB for ZGC in Java SE 25; compatibility and performance should still be verified for the actual runtime.
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Shenandoah
OpenJDK describes Shenandoah as reducing pauses by doing more work concurrently, including compaction, so pause times are no longer directly proportional to heap size. Its project documentation explains the design. The current OpenJDK command-line documentation identifies satb as single-generation and generational as generational.
Do not assume those modes or flags exist in every Java distribution. Check the documentation for the exact vendor, version, and build deployed, then confirm that the collector and requested mode are supported before adding flags. The OpenJDK command-line document is on the moving main branch, so its details can change.
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How to choose for your workload
- Start with the runtime default if your application meets its latency objective and you have no measured evidence that GC pauses are the bottleneck.
- Evaluate ZGC or Shenandoah if GC pauses materially contribute to tail-latency problems and the application can afford the concurrent CPU and memory costs.
- Compare ZGC with Shenandoah empirically only after confirming both are available in your deployed build and the modes being tested are supported.
- Make the decision against application behavior, not collector labels: heap size, live-set size, allocation rate, processor capacity, throughput needs, and tail-latency objectives all affect the result.
GC pause time is only one contributor to application response-time tails. A collector with short pauses may not improve p99 or p99.9 latency if another part of the system dominates, and its concurrent work may reduce throughput or consume CPU the application needs.
How to benchmark collectors fairly
Use the same JDK build, machine or container limits, application version, data set, heap settings, warm-up, and load profile for each run. Capture GC logs and application-level latency at the same time, and repeat representative tests rather than drawing conclusions from a single run.
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- Compare application latency at p95, p99, and p99.9 alongside the GC pause distribution; averages can hide damaging spikes.
- Measure throughput under a fixed CPU and memory budget, and record CPU used by GC and concurrent threads.
- Track live-set size, heap occupancy, allocation rate, and free headroom while collection is running.
- Record pause frequency and total time in collection, not only the longest pause.
- Exercise bursts, high promotion, and memory pressure; watch for full collections, allocation stalls or failures, and out-of-memory events.
For G1, use the logs to investigate symptoms rather than applying generic tuning recipes. Oracle’s tuning guide discusses humongous allocations, marking that starts too late, remembered-set work, and concurrent refinement. If you change heap sizing or the pause goal, change one variable at a time and rerun the same representative workload.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Practical recommendation
Use the default collector when it already meets the application’s latency and throughput requirements. If measurements show that GC pauses are hurting tail latency, test a concurrent low-latency option and judge the gain against throughput, GC CPU, and memory headroom. Choose between ZGC and Shenandoah only after checking the exact JDK build and comparing both under production-like conditions.
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