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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesJava garbage collection (GC) automatically reclaims heap memory occupied by objects the application can no longer reach. It still has performance costs: collection can pause application threads, use CPU while the application runs, affect throughput, and change the JVM’s memory footprint. There is no universally best collector. Choose a starting point based on whether your application prioritizes response time, throughput, or a small footprint, then validate it on the JDK and workload you actually deploy.
How does garbage collection work in Java?
The JVM manages the Java heap, where objects are allocated. When an object is no longer reachable by the application, the garbage collector can reclaim the space it occupies. Collectors use different strategies to find and reclaim that memory; some work happens during stop-the-world pauses, while some collectors also do work concurrently with application threads.
Automatic reclamation removes the need for most application code to free heap objects manually, but it does not make memory management cost-free. Stop-the-world work affects response time, concurrent work competes for CPU, and heap sizing affects the memory committed to the process. Evaluate GC against the service goal that matters: pause latency, throughput, or memory footprint.
Which Java garbage collector should you use?
Oracle’s JDK 25 collector-selection guidance offers starting points, not a cross-workload performance ranking. Results depend on heap size, live data, and available processor capacity.
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| Collector | Starting case in Oracle’s JDK 25 guidance | Tradeoff to consider |
|---|---|---|
| Serial | Small data set (about 100 MB or less), or one processor, when pauses are not a constraint. | A simple choice for small or single-processor cases; validate that it fits the workload. |
| Parallel | Peak application performance is the priority and pauses of a second or longer are acceptable. | Throughput-first; longer pauses may be acceptable in exchange for application performance. |
| G1 | Response time matters and shorter pauses are desired while maintaining throughput. | It does some work concurrently, which consumes resources otherwise available to the application; pause targets are not guarantees. |
| ZGC | Response time is a high priority. | Its low-latency design still needs adequate heap headroom and resources for concurrent collection. |
Oracle recommends first adjusting heap and generation sizing if the initial choice misses the application’s goal, then considering another collector. Make that decision from live behavior under representative load rather than assuming a collector label predicts performance.
Is G1 the default garbage collector?
Often, but do not assume it is the default on every deployed JVM. Oracle’s JDK 25 ergonomics guide describes G1 as the default on server-class machines and Serial otherwise. In that guide, a server-class machine has at least two processors and at least 1792 MB of physical memory. These documented selections are not universal sizing advice: runtime version, platform, containers, and explicit configuration can affect what your application uses. Confirm the collector on the actual runtime.
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The same JDK 25 documentation lists an initial heap selection of 1/64 and a maximum heap selection of 1/4 of physical memory. Treat those as documented defaults in that guide, not recommendations for setting heap sizes in every deployment.
What do GC pause and throughput settings mean?
GC tuning involves tradeoffs. Oracle documents -XX:MaxGCPauseMillis as a pause-time goal hint, not a hard limit. A shorter requested pause can lead the JVM to collect more frequently and reduce throughput; some applications cannot meet the requested goal at all. -XX:GCTimeRatio expresses a throughput goal, but heap sizing and the minimum amount of live data limit the JVM’s choices.
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Measure the application before tuning. Change a small number of relevant settings at a time and compare results under representative load; a flag that helps one workload may worsen another.
How does G1 behave, and does it guarantee a maximum pause?
G1 is generational and incremental. It tracks application and pause behavior to size collection work and selects regions with high expected reclaim efficiency. Some expensive work runs concurrently, but collection phases also include stop-the-world pauses. The concurrent work uses resources that the application could otherwise use.
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G1 aims to meet pause-time targets with high probability; it is not a real-time collector and does not guarantee a maximum pause for every event. A pause goal should therefore be treated as a tuning objective, not a service-level guarantee.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do you diagnose long G1 pauses or Full GC?
Start with GC logs and identify the event and its context before changing flags. Oracle’s JDK 25 troubleshooting guide describes log markers and diagnostic details that can narrow the cause.
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- Find Full GC and preceding evacuation failures. Look for
Pause Full (G1 Compaction Pause)and check earlier log entries for evacuation failures. Old-generation occupancy, marking that does not finish in time, and humongous allocations can contribute to Full GC. - Check for humongous regions. Use
gc+heap=infologging to inspect the humongous-region count. Oracle lists larger G1 regions or a larger heap as possible remedies, but the object-allocation pattern may need attention too. - Locate the work inside a pause. Use phase logging to see which phases account for pause time. Use
gc+cpu=infoto compare VM/user time, operating-system system time, and elapsed time. Memory operations, transparent huge pages, and log I/O can affect observed pauses. - Assess mixed collections that take too long. Oracle describes increasing
G1MixedGCCountTargetto spread reclamation across more collections. This can reduce the space reclaimed in the current cycle and may complicate sustained operation, so judge the change against the recurring pattern in the logs.
Do not respond to every long pause by indiscriminately increasing the heap or copying a flag set. Match a focused change to the diagnosed symptom, then compare behavior under representative load.
What should you know about ZGC in JDK 25?
Oracle’s JDK 25 tuning guide states that ZGC has been generational since JDK 24 and that the ZGenerational option was removed. Its documented ZGC heap range extends up to 16 TB; that is a stated range, not a promise of equivalent performance on every machine.
ZGC adapts settings including generations, GC thread counts, and tenuring thresholds. The main sizing task is to set a maximum heap large enough for the live set and the allocations that accumulate while concurrent collection is running. -Xmx sets the hard maximum; -XX:SoftMaxHeapSize can set a soft maximum below it.
Quick Recap
How should you begin GC tuning?
- Record the deployed JDK version, selected collector, heap limits, and relevant JVM options.
- Capture GC logs under representative load and identify the recurring symptom: long pauses, Full GC, throughput loss, or memory footprint.
- Use the log’s event markers, phase timings, heap details, and CPU/system-time information to form a cause-based diagnosis.
- Change one or a small number of relevant settings, then compare the same workload and service metrics.
- If the collector’s starting case still does not fit after heap and generation sizing, evaluate another collector against the application’s actual goals.
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