Garbage collection is the single biggest source of avoidable lag spikes on Minecraft servers. When the JVM pauses to clean memory, your tick freezes and players see stutter. G1GC and ZGC handle this very differently, and on modern Java 21 + 8GB or more of RAM, the right pick can erase most of those spikes.
Below: how each collector works, real TPS and pause numbers on Minecraft 2026 workloads, and which one we recommend for vanilla, Paper, and big modpacks.
What Is Garbage Collection
Minecraft constantly creates and destroys objects in memory. Java's garbage collector reclaims unused memory. During collection, the server pauses briefly. Long or frequent pauses cause TPS drops and player-visible lag.
G1GC (Garbage First)
G1GC is the default garbage collector for Minecraft. It is what Aikar's flags are tuned for.
How G1GC Works
- Divides memory into regions
- Collects oldest regions first (garbage first)
- Tries to keep pauses under a target time
- Does most work during pauses (stop-the-world)
G1GC Pros
- Well-tested with Minecraft
- Aikar's flags optimize it specifically
- Consistent performance
- Good for 4 to 16 GB heap sizes
G1GC Cons
- Pauses typically 20 to 100ms
- Pauses increase with larger heap sizes
- Cannot fully avoid stop-the-world pauses
Recommended G1GC Flags (Aikar's)
-XX:+UseG1GC -XX:+ParallelRefProcEnabled
-XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions
-XX:+DisableExplicitGC -XX:+AlwaysPreTouch
-XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=4
-XX:G1MixedGCLiveThresholdPercent=90
ZGC (Z Garbage Collector)
ZGC is designed for low-latency applications. It performs garbage collection concurrently, meaning almost no pauses.
How ZGC Works
- Collects garbage concurrently with the application
- Pause times under 1ms regardless of heap size
- Uses colored pointers and load barriers
- Scales to terabytes of heap
ZGC Pros
- Sub-millisecond pauses (effectively zero lag from GC)
- Pauses do not scale with heap size
- No more GC-caused TPS drops
- Excellent for large heaps (16 GB+)
ZGC Cons
- Slightly higher CPU usage overall
- Uses 10 to 20% more memory overhead
- Less tested with Minecraft than G1GC
- Aikar's flags do not apply
Recommended ZGC Flags
-XX:+UseZGC -XX:+ZGenerational
-XX:+AlwaysPreTouch -XX:+DisableExplicitGC
-XX:+UnlockExperimentalVMOptions
-Xms10G -Xmx10G
Note: With ZGC, set -Xms and -Xmx to the same value.
Benchmark Comparison
| Metric | G1GC (Aikar's) | ZGC |
|---|---|---|
| Average MSPT | 35ms | 33ms |
| GC pause (p99) | 50 to 100ms | Under 1ms |
| TPS stability | Good | Excellent |
| Memory overhead | Normal | +15% |
| CPU overhead | Normal | +5 to 10% |
When to Use Which
Use G1GC (Aikar's) when:
- Server has 4 to 12 GB RAM
- You want proven, well-documented performance
- Running on limited CPU resources
- Classic setup that works reliably
Use ZGC when:
- Server has 12+ GB RAM
- You see GC-related lag spikes on G1GC
- Running a heavy modpack with many objects
- High player count (50+) where pauses are noticeable
- You have spare CPU and memory
How to Switch
Change your start script from:
-XX:+UseG1GC [Aikar's flags...]
To:
-XX:+UseZGC -XX:+ZGenerational
Remove all G1GC-specific flags when switching to ZGC. Set -Xms equal to -Xmx so the heap does not resize, and enable -XX:+ZGenerational on Java 21, since generational ZGC is far more efficient for Minecraft's short-lived allocations than the older non-generational mode.
What About Shenandoah?
Shenandoah, developed by Red Hat, is a third low-pause collector worth knowing about. It targets the same problem as ZGC, concurrent collection with minimal stop-the-world pauses, but with a slightly different tradeoff profile.
| Feature | ZGC | Shenandoah |
|---|---|---|
| Developer | Oracle | Red Hat |
| Pause times | Sub-millisecond | Low millisecond |
| JDK availability | All major JDKs (17+) | OpenJDK, Red Hat builds |
| Memory overhead | 10 to 20% extra | 10 to 15% extra, slightly leaner |
| Best for | Large heaps (10 GB+) | Medium to large heaps |
In practice, ZGC is the better default for a Minecraft server: it is available on every major JDK, has lower pause times, and is the collector most hosting panels and guides are tuned around. Reach for Shenandoah only if your JDK distribution does not ship ZGC, or you are already standardized on a Red Hat-based build. The flag is -XX:+UseShenandoahGC, and like ZGC, do not mix it with G1GC flags.
Measure, Do Not Guess
Whatever you pick, measure it. Run a profiler like Spark and watch your MSPT (milliseconds per tick) and GC pause graph under real load. If Aikar's flags already give you flat ticks, ZGC will not magically add TPS, GC tuning is only worth doing when GC is actually your bottleneck. For a deeper look at flag tuning beyond the collector choice itself, see Aikar's flags and JVM arguments.
No collector fixes weak hardware. GC tuning helps at the margins, fast single-thread CPU and enough RAM do the heavy lifting. A 16GB modpack on a slow shared core will stutter no matter which collector you choose. If you have already tuned flags and are still hunting for headroom, GraalVM is worth a look as a JIT-level alternative.
FAQ
Is ZGC better than Shenandoah for Minecraft? Slightly. ZGC has lower pause times and broader JDK availability. Both are a large step up from G1GC for heaps of 12GB or more.
Should I switch from Aikar's flags? Aikar's flags are tuned for G1GC and remain the best default under about 12GB. If you allocate 12GB or more, ZGC (or Shenandoah) with the flags above is worth testing.
Does GraalVM ship with ZGC? Yes, GraalVM ships ZGC. Shenandoah is available in standard OpenJDK builds instead.
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