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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchAikar’s flags are Java startup arguments that tune the G1 garbage collector for Minecraft Java servers, especially Paper and other Spigot-family software. They can reduce garbage-collection pause spikes, but they cannot compensate for insufficient CPU, RAM, unsuitable Java versions, or overloaded worlds. Use the current flag block from the official PaperMC Aikar’s flags documentation, set equal -Xms and -Xmx values, and reserve memory for the operating system.
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What Aikar’s flags actually are
Aikar’s flags are a coordinated set of Java Virtual Machine (JVM) arguments for running a Minecraft Java server. They primarily configure G1GC, Java’s Garbage-First garbage collector, around the allocation pattern produced by Minecraft. The flags were published by PaperMC contributor Daniel Ennis, known online as Aikar, and are documented by PaperMC.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsMinecraft creates and discards large numbers of short-lived Java objects while loading chunks, simulating entities, processing redstone, handling plugins, and serving players. If the garbage collector waits too long or performs a large collection, the server thread can stop for seconds. Players experience that stop as a tick-time spike, rubber-banding, delayed block updates, or a temporary freeze. The purpose of the flag set is to make collection work more predictably; it is not a general-purpose “lag fix.”
The authoritative, version-sensitive reference is PaperMC’s Aikar’s flags page. Flag strings copied from old guides can differ from the current recommendation, and some community examples add system properties such as -Dusing.aikars.flags or -Daikars.new.flags. Those properties are not a substitute for checking the current PaperMC block.
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When these flags are a sensible choice
- You run a Java Edition server based on Paper or another Spigot-family implementation.
- Your Java runtime is supported by the Minecraft version and server software.
- The machine has enough physical memory and CPU capacity for the player count, view distance, simulation distance, plugins, and mods you actually use.
- You can restart the server to change JVM options; flags are startup settings, not commands that can be safely applied to a running process.
They are less useful when the dominant problem is a saturated CPU core, slow disk access during chunk generation, a faulty plugin, excessive entity counts, network congestion, or a mod that performs expensive work on the main thread. Diagnose those constraints instead of continually increasing the heap or adding more flags.
Choose Java before choosing flags
The Java major version must match the Minecraft release and the server software’s support policy. Check the release notes for your exact Minecraft and Paper build, then verify the runtime with:
java -versionOne third-party compatibility guide associates Java 21 with Minecraft 1.20.5 through 1.21.x and Java 25 with Minecraft 26.1. Those pairings are not a replacement for the current Paper or Minecraft requirements; use them only as leads and confirm the required version before installing or changing Java. Running an unsupported runtime can produce an immediate startup error, class-file errors, or subtle plugin incompatibilities.
Plan memory correctly
-Xmx is the maximum Java heap and -Xms is the initial heap. For this flag set, keep them equal so the JVM reserves one stable heap instead of repeatedly resizing it:
-Xms8G -Xmx8GThe heap is not the server’s total memory footprint. Java’s native allocations, thread stacks, direct buffers, libraries, the operating system, and other services also need RAM. Paper’s Aikar’s flags guidance has stated that servers should have at least 6–10 GB available, while an older mirrored revision advises reducing the heap values by roughly 1,000–1,500 MB to leave operating-system headroom. Treat those figures as guidance rather than a universal minimum, and recheck the live PaperMC page because wording can change.
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| Physical RAM in the machine | Illustrative equal heap setting | What the remainder must cover |
|---|---|---|
| 8 GB | About 6–7 GB | Operating system, Java native memory, background services, and filesystem cache |
| 12 GB | About 10–11 GB | Operating system plus any panel, proxy, database, or monitoring process |
| 16 GB | About 14–15 GB | Operating system and other workloads; lower the heap if the host is shared |
These are planning examples, not guarantees. A heavily modded world or a large render distance may need more heap, while a small vanilla server may run comfortably with less. If the host begins swapping, the server can become slower even though the Java heap is not full. If the operating system kills Java for lack of memory, reduce both -Xms and -Xmx by the same amount.
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Get the current flag block
- Open the PaperMC Aikar’s flags page.
- Identify the command block that matches your server’s memory range and current documentation. Do not assume a string from a blog, video, or generator is still current.
- Copy the complete block, preserving every
-XX:,-Xlog:, and system-property argument exactly as shown. - Place your equal
-Xmsand-Xmxvalues in the positions specified by the documentation, then keep the Paper command’s-jarand--noguiarguments.
Some community guides describe a larger-young-generation variant for heaps above 12 GB. That is a third-party variation, not a reason to substitute an old command for the version on PaperMC’s page.
Install the flags on Linux or other Unix-like systems
Create or edit a startup script in the directory containing your Paper server JAR. The example below shows the structure; obtain the actual Aikar block from PaperMC rather than treating the comment as a flag recipe.
#!/usr/bin/env bash
cd -- "$(dirname -- "$0")"
exec java -Xms8G -Xmx8G
# add the current Aikar flag block from PaperMC here
-jar paper.jar --noguiBecause a shell comment cannot appear in the middle of a continued command, remove the comment line when you paste the real flags. A practical final layout is:
#!/usr/bin/env bash
cd -- "$(dirname -- "$0")"
exec java -Xms8G -Xmx8G [current PaperMC flags] -jar paper.jar --noguiIn a real script, replace the bracketed notation with the literal arguments from PaperMC; do not leave the brackets in place. Save the file as start.sh, make it executable, and launch it:
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chmod +x start.sh
./start.shOn the first run, Paper may create configuration files or stop to ask you to accept the Minecraft EULA. Complete that one-time setup, then run the same script again. Keep the script under version control or make a backup so a later edit does not silently remove a required argument.
Install the flags on Windows
Use a batch file in the folder containing paper.jar. The command must remain on one line unless you use Windows batch continuation syntax.
@echo off
cd /d "%~dp0"
java -Xms8G -Xmx8G [current PaperMC flags] -jar paper.jar --nogui
pauseReplace the bracketed notation with the literal current arguments from PaperMC, save the file as start.bat, and double-click it or run it from Command Prompt. The final pause keeps an error message visible after an immediate exit; remove it when a hosting panel or service manager handles process supervision.
What the commonly documented options control
The exact set and values can change, so regard the following as explanations of options commonly associated with Aikar-style configurations, not a replacement command line.
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|---|---|---|
-XX:MaxGCPauseMillis=200 |
Sets a target pause time for G1GC’s balancing decisions. | It is a goal, not a promise that every pause will be 200 ms or less. Forcing an aggressive target can increase background collection work. |
-XX:+AlwaysPreTouch |
Touches committed heap pages during startup. | Startup takes longer and memory is committed up front, but runtime page faults can be more predictable. |
-XX:+DisableExplicitGC |
Prevents application code from directly requesting a full collection with System.gc(). |
Useful when a plugin makes harmful explicit requests; a plugin that relies on such a request may behave differently. |
-XX:MaxTenuringThreshold=1 |
Controls how many young-generation collections an object can survive before promotion. | Changes object-promotion behavior; retain it only when it appears in the current official block for your Java version. |
-XX:G1HeapRegionSize=8M |
Sets the size of G1’s heap regions. | Region sizing is tied to heap layout and workload. Do not copy this value blindly to a different heap size or Java release. |
Seeing one of these options in a guide does not prove that the guide is current. Java releases can deprecate, ignore, or reinterpret options, and Paper may revise the recommended combination.
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Confirm that the server is using the intended settings
- Start the server from the script rather than an old panel command. Check the process command line in your operating system or hosting panel and verify that both heap values and the current PaperMC arguments are present.
- Read the first screenful of Java output. An unrecognized-option or deprecated-option message identifies a mismatch between the flag block and your Java version.
- Join with a test client and watch tick time, not just the number of online players. A stable server generally keeps its tick-time distribution within the 50 ms budget needed for 20 TPS, but workload spikes can still occur.
- Repeat the observation during the activity that previously caused pauses: teleporting through new terrain, a farm activation, a world save, or a scheduled plugin task.
Do not claim a performance percentage from the flags alone. No independent primary-source statistic establishing a universal TPS increase or a fixed reduction in lag was identified. Results depend on Java version, heap size, hardware, world data, plugins, mods, and player behavior.
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The process exits with an invalid or unrecognized option
Compare the command character by character with the current PaperMC page, then check java -version. Remove obsolete options only after confirming that the current documentation no longer uses them; do not mix fragments from multiple guides. A Java major-version change is a common cause.
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The operating system kills Java or the host starts swapping
The heap is too close to total physical memory, or another service is consuming the remainder. Stop the server, lower both -Xms and -Xmx by the same amount, and leave roughly 1,000–1,500 MB or more for the host as appropriate. On a shared machine, account for the panel, proxy, database, backups, and monitoring agents.
Startup takes longer after adding the flags
-XX:+AlwaysPreTouch can make startup spend time committing heap pages. That delay is expected when the option is present. If a small server does not benefit from the longer startup, use the current PaperMC recommendation rather than retaining an old custom variant.
Pauses continue during exploration or busy farms
Measure the workload instead of increasing memory indefinitely. Check CPU saturation, entity counts, chunk-generation cost, simulation and view distances, plugin tasks, mod behavior, and storage latency. A larger heap can defer collections while making individual collections more expensive, and it cannot repair a main-thread bottleneck.
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Performance became worse after changing Java or mods
Record the previous Java version, Paper build, flags, heap, and mod/plugin set. Roll back one change at a time, then retest the same in-game scenario. Keep the flag block matched to the runtime; a configuration tuned for one Java release is not automatically valid for another.
G1GC versus ZGC: should you switch?
Aikar’s flags are a G1GC configuration. Some third-party guides discuss generational ZGC for modded servers or very large heaps, but that is a workload-specific alternative rather than an automatic upgrade. ZGC and G1GC have different throughput, latency, memory, and compatibility trade-offs, and the best choice depends on the Java release and server software.
| Decision factor | G1GC with Aikar’s flags | ZGC |
|---|---|---|
| Documentation path | PaperMC publishes a dedicated Aikar’s flags configuration. | Use the Java and server software documentation for a ZGC command; do not transplant G1 arguments. |
| Typical reason to consider it | Predictable collection behavior for common Paper workloads. | Testing very large heaps or latency-sensitive modded workloads where the runtime supports it. |
| Evidence needed | Compare pauses and tick-time data before and after the change. | Run the same controlled test; third-party “TPS boost” headlines are not independent benchmarks. |
Change collectors only after establishing a baseline, and keep a rollback command. A collector switch cannot compensate for an undersized CPU, insufficient total RAM, or an overloaded world.
Maintain the configuration as Minecraft changes
Flag strings, Java requirements, and Paper recommendations can change with Minecraft and JVM releases. Recheck the official page when upgrading Minecraft, changing Java, changing Paper’s major line, or moving to a substantially different heap size. A six-month review interval is a reasonable operational reminder, and the documented freshness guidance for this topic is 180 days.
Keep these details together in your server notes:
- Minecraft, Paper, and Java versions.
- Physical RAM and the equal
-Xms/-Xmxvalues. - The exact startup command and date it was copied from PaperMC.
- Player limit, view distance, simulation distance, mods, plugins, and proxy services.
- Observed tick-time or pause symptoms before and after a change.
That record makes a rollback possible and prevents a future administrator from combining a current heap setting with an obsolete flag string.
The practical verdict
Use Aikar’s flags when you run a supported Paper-family server and want a documented G1GC configuration. Copy the current block from PaperMC, match Java to Minecraft, set equal heap limits, and reserve memory for everything outside the Java heap. Treat the flags as one part of capacity planning and diagnosis—not as a promise of a particular TPS number or a cure for hardware, world, plugin, or mod bottlenecks.
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