Mastering JVM Garbage Collectors: Which One Fits Your Application?

This article introduces the main JVM garbage collectors, showing diagrams for each and explaining their characteristics.

Serial (Serial Collector) : Used for young generation GC with a copying algorithm; runs single‑threaded by default in Client mode, leading to longer stop‑the‑world pauses on multi‑core CPUs.

ParNew (Parallel Collector) : Similar to Serial but typically used for parallel young‑generation collection and runs in Server mode; performance may be lower than Serial on single‑core CPUs.

Parallel Scavenge (Parallel Young) : Young‑generation collector using a copying algorithm with parallel threads; focuses on throughput control, e.g., achieving 99% throughput when GC time is 1 min of 100 min total runtime.

Serial Old (Serial Collector for Old Generation) : Old‑generation version of Serial, using a mark‑compact algorithm, mainly for JVMs started in Client mode.

Parallel Old (Parallel Collector for Old Generation) : Old‑generation version of Parallel, also using a mark‑compact algorithm.

CMS (Concurrent Mark‑Sweep) : A concurrent collector aiming for minimal pause times, suitable for B/S architectures; uses a mark‑sweep algorithm, which can cause memory fragmentation. Its phases are Initial Mark → Concurrent Mark → Remark → Concurrent Sweep.

G1 Collector : A server‑oriented collector that may replace CMS. Its key features include:

a) Concurrency and parallelism: utilizes multiple CPU cores to reduce stop‑the‑world pauses while running alongside application threads. b) Region‑based collection: maintains generational concepts, managing regions independently based on object lifetimes. c) Space efficiency: employs a mark‑compact algorithm, minimizing memory fragmentation for long‑running applications. d) Predictable pauses: offers shorter and more predictable pause times compared to CMS.

G1’s operation (without counting the Remembered Set) proceeds through Initial Mark → Concurrent Mark → Final Mark → Live Data Counting and Evacuation.

GC Parameter Summary

-XX:+<option> enables the option.

-XX:-<option> disables the option.

-XX:<option>=<value> sets the option to a numeric or string value.

-XX:+UseSerialGC: use Serial collector for both young and old generations.

-XX:SurvivorRatio: set the ratio between Eden and Survivor spaces.

-XX:NewRatio: set the ratio between young and old generations.

-XX:+UseParNewGC: use ParNew for the young generation.

-XX:+UseParallelGC: use Parallel Scavenge for the young generation.

-XX:+UseParallelOldGC: use Parallel Old for the old generation.

-XX:ParallelGCThreads: specify the number of GC threads.

-XX:+UseConcMarkSweepGC: use CMS (with Serial Old for the old generation).

-XX:ParallelCMSThreads: set the number of CMS threads.

-XX:CMSInitiatingOccupancyFraction: trigger CMS when old‑gen usage reaches this percentage.

-XX:+UseCMSCompactAtFullCollection: compact memory after a full CMS collection.

-XX:CMSFullGCsBeforeCompaction: perform memory compaction after this many CMS cycles.

-XX:+CMSClassUnloadingEnabled: allow class metadata to be reclaimed.

-XX:CMSInitiatingPermOccupancyFraction: start CMS when the permanent generation reaches this percentage.

-XX:UseCMSInitiatingOccupancyOnly: trigger CMS only when the occupancy threshold is reached.

GC Parameter Combinations

-XX:+UseSerialGC → Serial (young) / Serial Old (old)

-XX:+UseParallelGC → Parallel Scavenge (young) / Serial Old (old)

-XX:+UseConcMarkSweepGC → ParNew (young) / CMS (old)

-XX:+UseParNewGC → ParNew (young) / Serial Old (old)

-XX:+UseParallelOldGC → Parallel Scavenge (young) / Parallel Old (old)

-XX:+UseConcMarkSweepGC -XX:+UseParNewGC → Serial (young) / CMS (old)

-XX:+UnlockExperimentalVMOptions -XX:+UseG1GC → G1 (both young and old)