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The safe way to use them is to copy the current flag set from the official PaperMC Aikar’s flags page, run a Java version compatible with your Minecraft release, set -Xms and -Xmx to the same value, and reserve memory for the operating system. The exact string changes over time; treat the PaperMC page as the authority rather than copying an old guide.
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What Aikar’s flags change
Minecraft constantly creates and discards Java objects while it loads chunks, processes entities, handles plugins and mods, and serves players. Java’s garbage collector eventually reclaims those objects. With default G1GC settings, a busy server can occasionally spend several seconds collecting memory. During that stop, the main server thread cannot keep up with ticks, and players experience rubber-banding, delayed block updates, or a sudden drop in responsiveness.
Aikar’s flags adjust G1GC’s heap layout, pause target and object-promotion behavior for this allocation pattern. They are startup options, not a plugin and not a setting inside the Minecraft server console. The flags are associated with PaperMC contributor Daniel Ennis, known as Aikar, and are primarily used with Paper and other Spigot-family servers.
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They should be viewed as a garbage-collection configuration baseline. A healthy result is steadier behavior under allocation pressure, not a guaranteed increase in average TPS. No independent primary-source study establishing a universal TPS-percentage improvement was identified, so claims of a fixed “TPS boost” should be treated as marketing rather than a measured promise.
Before you install them
Match Java to the Minecraft release
Use a Java runtime supported by the exact Minecraft and server version you run. Java compatibility is a prerequisite: flags cannot correct an incompatible or outdated runtime. One third-party guide recommends Java 21 for Minecraft 1.20.5 through 1.21.x and Java 25 beginning with Minecraft 26.1, but that recommendation was not confirmed by a primary source. Check the release notes for your server software and the Java requirement for your Minecraft version before changing runtimes.
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java -versionRun this in the same account and environment that starts the server. If it reports a different Java installation than the one you intended, use the full path to the desired executable in your script or correct the system PATH first.
Confirm that the server software is appropriate
Aikar’s flags are designed around the allocation behavior of Minecraft Java servers and are most commonly documented for Paper and Spigot-family software. They are not a universal recipe for every Java game server. Confirm that your server’s documentation supports G1GC and these arguments before applying them to a different implementation.
Plan RAM for the whole machine
The heap is only one consumer of memory. The operating system, Java’s native memory, file caches, map tools, databases, monitoring agents and other services also need headroom. The PaperMC Aikar’s flags page has stated a broad recommendation of at least 6–10 GB of RAM regardless of player count; the value came from a search snippet and may change as the page is revised, so verify the current wording when you size a new server.
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An older PaperMC documentation revision mirrored on GitHub advises reducing the values you would otherwise give the heap by about 1,000–1,500 MB to avoid an operating-system out-of-memory kill. That is a practical headroom guideline, not a guarantee for every machine. Leave more room if the host runs a graphical desktop, containers, backups or other services.
How to obtain the current flag string
Do not treat a blog post or a generated command as canonical. Open PaperMC’s Aikar’s flags documentation and copy the complete, current string for your server’s Java and memory setup. Flag names and values can change as Minecraft, Java and Paper evolve.
Third-party guides sometimes add system properties such as -Dusing.aikars.flags or -Daikars.new.flags, or differ by a small option. Those additions are not evidence that the string is current. Compare every argument with the PaperMC page and remove anything you cannot explain or verify.
Set heap values correctly
-Xms sets the initial Java heap and -Xmx sets its maximum. Set both to the same amount so the JVM does not repeatedly resize the heap while the server is running. Choose a value that fits after reserving operating-system headroom; do not assign all physical RAM to -Xmx.
| Setting | Meaning | Safe practice |
|---|---|---|
-Xms |
Heap size reserved when Java starts | Make it equal to -Xmx |
-Xmx |
Maximum Java heap | Reduce it to leave roughly 1,000–1,500 MB or more for the operating system and other processes, depending on the host |
| Physical RAM | Total memory installed or assigned to the machine | Never assume all of it is available to the Minecraft heap |
For example, a machine with 16 GB of physical RAM should not automatically receive -Xmx16G. A smaller equal pair such as -Xms14G -Xmx14G may still be too aggressive if other services are active; the correct number depends on the complete workload. The numbers in this example illustrate the relationship between the options, not a universal sizing rule.
Linux setup: a repeatable startup script
Keep the server jar, world data and script in one directory, and make a backup before changing startup options. The following structure keeps the flag string in one variable so it can be replaced when PaperMC updates the recommended set:
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#!/usr/bin/env bash
set -euo pipefail
JAVA_BIN="java"
HEAP="8G"
AIKAR_FLAGS="paste-the-current-string-from-PaperMC-here"
exec "$JAVA_BIN" -Xms"$HEAP" -Xmx"$HEAP" $AIKAR_FLAGS -jar paper.jar --noguiIn this example, replace the value of AIKAR_FLAGS with the arguments copied from the official page; do not include another -Xms or -Xmx in that variable. If the official string already includes memory options, remove those duplicates and keep one equal pair in your script.
- Save the file as
start.shin the server directory. - Give it execute permission with
chmod +x start.sh. - Run it with
./start.shand watch the first lines of output for Java errors or unrecognized options. - Stop the server normally, then restart it once more to confirm that the same script is used after a reboot.
Shell quoting matters. The official string is a sequence of separate JVM arguments, so do not wrap the entire string in one pair of quotes. If your distribution uses a service manager, put the same command in that service’s documented startup field rather than running a second copy from a terminal.
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On Windows, create a start.bat file in the server directory. Keep the heap values equal and paste the current PaperMC arguments after them:
@echo off
set "HEAP=8G"
set "AIKAR_FLAGS=paste-the-current-string-from-PaperMC-here"
java -Xms%HEAP% -Xmx%HEAP% %AIKAR_FLAGS% -jar paper.jar --nogui
pauseOpen the batch file from the server directory so relative paths resolve to the correct jar and world. If Windows reports that an option is unrecognized, check for a typo, an argument copied from a different Java version, or a stale flag set from an older guide. Keep a copy of the previous working script so you can restore it quickly.
What the commonly listed options do
The following options are frequently associated with Aikar’s configuration in third-party explanations. Their presence and exact values must be checked against the current PaperMC page before use; the table is an explanation of purpose, not a replacement for the official string.
| Option | Purpose | Important qualification |
|---|---|---|
-XX:MaxGCPauseMillis=200 |
Gives G1GC a target pause time in milliseconds. | The value is a target, not a promise that every collection will finish within 200 ms. |
-XX:+AlwaysPreTouch |
Touches the requested heap pages during startup so memory is committed before normal play. | Startup can take longer, and the machine must have enough available memory. |
-XX:+DisableExplicitGC |
Prevents application code from directly requesting a full System.gc() collection. |
It does not stop normal automatic garbage collection. |
-XX:MaxTenuringThreshold=1 |
Controls how many collection cycles an object may survive before promotion. | Changing the value independently can alter allocation behavior; use the tested set rather than tuning one flag in isolation. |
-XX:G1HeapRegionSize=8M |
Sets the size of G1 heap regions. | Region sizing interacts with total heap size, so verify that the value is still recommended for your release. |
Do not add an option merely because its name appears in an old command. G1GC has evolved with the JDK, and a flag that was useful for one Java release may be unnecessary, ignored or rejected in another.
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First launch checks
After starting the server with the new command, verify the basics before inviting players:
- The console reaches the normal “Done” state without an “Unrecognized VM option” or startup failure.
- The reported Java version is the one you intended to use.
- The process stays within the machine’s memory budget instead of pushing the operating system into swapping or an out-of-memory kill.
- The server can load the world, enable plugins or mods, and save normally.
- Your monitoring shows that the heap is not permanently at its limit during ordinary activity.
Always test with the same world, plugins, mods and view distance that create the real workload. A quiet test server can start successfully while a production server still lacks CPU time or memory.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Diagnosing problems after applying the flags
The JVM will not start
Read the first error line, not only the final shutdown message. An unrecognized option usually means the copied string targets another Java release or contains a typographical error. Remove the new arguments, confirm that the unmodified server starts, then compare the current PaperMC string option by option. Do not mix fragments from several guides.
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The machine runs out of memory
Lower both -Xms and -Xmx by the same amount and retain operating-system headroom. Check for other Java processes, map renderers, backups, containers and desktop services. A larger heap is not automatically better: if the host cannot keep it resident, the resulting swapping or kill is worse than a smaller, stable heap.
Lag continues despite successful startup
Garbage collection is only one possible source of lag. Profile the server’s tick time and inspect CPU saturation, entity counts, chunk generation, redstone activity, plugin behavior, mod requirements and network conditions. Aikar’s flags cannot compensate for an undersized CPU, insufficient total RAM or player and mod loads beyond what the machine can sustain.
Startup is noticeably slower
An option such as -XX:+AlwaysPreTouch can make Java touch a large heap before the server accepts connections. That trades some startup time for more predictable memory commitment during operation. Do not interrupt startup simply because it takes longer; check whether the process eventually reaches the normal ready state and whether the host has enough free memory.
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Large heaps and alternative collectors
Some third-party guides describe a variant with a larger young generation for servers above 12 GB. That is not a universal rule and should not be adopted without checking the current PaperMC documentation and measuring the result on your workload.
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Other guides compare G1GC with generational ZGC, particularly for heavily modded servers or very large heaps. Those comparisons are third-party advice, not a guarantee that ZGC will outperform G1GC on your machine. Changing collectors changes the tuning model and should be treated as a controlled experiment: back up the server, change one variable, reproduce the same workload, and compare pause behavior, tick time and memory pressure. If the result is worse, return to the documented configuration that was stable.
How to keep the configuration current
Flag strings and Java recommendations are version-sensitive. Recheck the official PaperMC page when you upgrade Minecraft, switch Paper builds, change Java major versions or move to a machine with a different memory size. A practical review interval is at least every six months, and immediately after a major Minecraft or Java release.
- Record the Minecraft version, Paper build, Java version and heap values beside your startup script.
- Keep the previous script and a recent world backup before changing arguments.
- Copy the complete current string instead of appending new options to an old one.
- Remove duplicate memory settings and any property whose purpose you cannot verify.
- Re-test during the busiest expected workload, not only during an empty-server startup.
Generators and calculators can produce a starting command from a RAM value, but they are secondary references. Use them only to understand formatting, then compare their output with the official PaperMC documentation. No generator can know your CPU limits, plugin behavior, mod mix or the memory required by other services.
What Aikar’s flags can—and cannot—solve
| Situation | Likely effect of the flags | What you still need to address |
|---|---|---|
| Long pauses caused by G1GC collecting many short-lived objects | They can provide a more suitable G1GC policy and reduce avoidable pause behavior. | Measure pauses and tick time under the real workload. |
| Not enough physical RAM | They cannot create memory; an oversized heap can trigger an operating-system kill. | Reduce the heap or add capacity, while preserving headroom. |
| CPU saturation or slow single-thread performance | They do not make the server thread faster. | Reduce expensive activity, improve hardware or adjust player and mod limits. |
| A plugin or mod causing excessive work | They do not repair faulty application behavior. | Profile, update or remove the offending component. |
| Network congestion or a distant connection | They do not reduce latency on the network path. | Investigate routing, bandwidth and client-side conditions. |
Bottom line for server owners
Use Aikar’s flags as a documented G1GC baseline, not as a magic lag fix. Match Java to Minecraft, copy the current arguments from PaperMC, set equal -Xms and -Xmx values, reserve roughly 1,000–1,500 MB or more for the rest of the machine, and verify the result under real player and mod loads. When lag remains, investigate CPU, memory pressure and server workload instead of adding more undocumented JVM options.
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