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Update Aikar's flags in StartScriptGenerator #828

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bb5c6dd
Update Aikar's flags in StartScriptGenerator
DajMiodu Nov 7, 2026
0125331
Update StartScriptGenerator.svelte
DajMiodu Nov 7, 2026
2a5effc
Updated aikars-flags site
DajMiodu Nov 7, 2026
7b154a3
Updated the flag explanation
DajMiodu Nov 7, 2026
0b2857d
oops, fixes
DajMiodu Nov 7, 2026
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8 changes: 3 additions & 5 deletions src/components/tools/StartScriptGenerator.svelte
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Original file line number Diff line number Diff line change
Expand Up @@ -9,27 +9,25 @@
const flags: Option[] = [
{ id: "none", label: "None", value: "" },
{
id: "aikar",
label: "Aikar's",
id: "paper",
label: "Paper",
value: [
"-XX:+AlwaysPreTouch",
"-XX:+DisableExplicitGC",
"-XX:+ParallelRefProcEnabled",
"-XX:+PerfDisableSharedMem",
"-XX:+UnlockExperimentalVMOptions",
"-XX:+UseG1GC",
"-XX:G1HeapRegionSize=8M",
"-XX:G1HeapWastePercent=5",
"-XX:G1MaxNewSizePercent=40",
"-XX:G1MixedGCCountTarget=4",
"-XX:G1MixedGCLiveThresholdPercent=90",
"-XX:G1NewSizePercent=30",
"-XX:G1RSetUpdatingPauseTimePercent=5",
"-XX:G1ReservePercent=20",
"-XX:InitiatingHeapOccupancyPercent=15",
"-XX:MaxGCPauseMillis=200",
"-XX:MaxTenuringThreshold=1",
"-XX:SurvivorRatio=32",
"-XX:+UseCompactObjectHeaders",
"-Dusing.aikars.flags=https://mcflags.emc.gs",
"-Daikars.new.flags=true",
"" /* space */,
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114 changes: 52 additions & 62 deletions src/content/docs/paper/admin/how-to/aikars-flags.md
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Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,7 @@ our **[Script Generator](/misc/tools/start-script-gen)**.
:::

```bash
java -Xms10G -Xmx10G -XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC -XX:+AlwaysPreTouch -XX:G1NewSizePercent=30 -XX:G1MaxNewSizePercent=40 -XX:G1HeapRegionSize=8M -XX:G1ReservePercent=20 -XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=4 -XX:InitiatingHeapOccupancyPercent=15 -XX:G1MixedGCLiveThresholdPercent=90 -XX:G1RSetUpdatingPauseTimePercent=5 -XX:SurvivorRatio=32 -XX:+PerfDisableSharedMem -XX:MaxTenuringThreshold=1 -Dusing.aikars.flags=https://mcflags.emc.gs -Daikars.new.flags=true -jar paper.jar --nogui
java -Xms10G -Xmx10G -XX:+AlwaysPreTouch -XX:+DisableExplicitGC -XX:+PerfDisableSharedMem -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1HeapRegionSize=8M -XX:G1MaxNewSizePercent=40 -XX:G1MixedGCCountTarget=4 -XX:G1MixedGCLiveThresholdPercent=90 -XX:G1NewSizePercent=30 -XX:G1RSetUpdatingPauseTimePercent=5 -XX:G1ReservePercent=20 -XX:InitiatingHeapOccupancyPercent=15 -XX:MaxTenuringThreshold=1 -XX:SurvivorRatio=32 -XX:+UseCompactObjectHeaders -Dusing.aikars.flags=https://mcflags.emc.gs -Daikars.new.flags=true -jar server.jar --nogui
```

:::danger[Do not allocate all of your available memory on a shared host!]
Expand Down Expand Up @@ -80,10 +80,28 @@ up much space (5MB)
If it does not need the memory given to it, it will not use it. The entire engine operates
differently and does not suffer from too large of heaps, and this is industry-wide accepted
information that under G1 to keep Xms and Xmx the same!

2. **AlwaysPreTouch:** AlwaysPreTouch gets the memory setup and reserved at process start ensuring
it is contiguous, improving the efficiency of it more. This improves the operating systems
memory access speed. Mandatory to use Transparent Huge Pages

3. **+DisableExplicitGC:** Many plugins think they know how to control memory, and try to invoke
garbage collection. Plugins that do this trigger a full garbage collection, triggering a massive
lag spike. This flag disables plugins from trying to do this, protecting you from their bad
code.

4. **+PerfDisableSharedMem:** Causes GC to write to file system which can cause major latency if
disk IO is high - see https://www.evanjones.ca/jvm-mmap-pause.html

2. **UnlockExperimentalVMOptions:** needed for some the below options
5. **UnlockExperimentalVMOptions:** needed for some the below options

6. **G1HeapRegionSize=8M+:** Default is auto calculated. Super important for Minecraft, especially
1.15, as with low memory situations, the default calculation will in most times be too low. Any
memory allocation half of this size (4MB) will be treated as "Humongous" and promote straight to
old generation and is harder to free. If you allow Java to use the default, you will be
destroyed with a significant chunk of your memory getting treated as Humongous.

3. **G1NewSizePercent:** These are the important ones. You now can specify percentages of an
7. **G1NewSizePercent:** These are the important ones. You now can specify percentages of an
overall desired range for the new generation. With these settings, we tell G1 to not use its
default 5% for new gen, and instead give it 40%! **Minecraft has an extremely high memory
allocation rate**, ranging to at least 800MB/second on a 30 player server! And this is
Expand All @@ -97,75 +115,47 @@ up much space (5MB)
we are able to slow down the intervals of young gen collections, resulting in more time for
short-lived objects to die young and overall more efficient GC behavior.

4. **G1MixedGCLiveThresholdPercent:** Controls when to include regions in mixed GCs in the young
GC collection, keeping old gen tidy without doing a normal old gen GC collection. When your
memory is less than this percent, old gen won't even be included in 'mixed' collections. Mixed
are not as heavy as a full old collection, so having small incremental cleanups of old keeps
memory usage light.

Default is 65 to 85 depending on the Java version, we are setting that to 90 to ensure we reclaim garbage
in old gen as fast as possible to retain as much free regions as we can.

5. **G1ReservePercent=20:** Minecraft memory allocation rate in up-to-date versions is really insane. We
run the risk of a dreaded "to-space exhaustion" not having enough memory free to move data
around. This ensures more memory is waiting to be used for this operation. Default is 10, so we
are giving another 10 to it.

6. **MaxTenuringThreshold=1:** Minecraft has a really high allocation rate of memory. Of that
memory, most is reclaimed in the eden generation. However, transient data will overflow into
survivor. Initially played with completely removing survivor and had decent results, but does
result in transient data making its way to old which is not good. Max Tenuring 1 ensures that we
do not promote transient data to old generation, but anything that survives 2 passes of GC is
just going to be assumed as longer-lived.
8. **G1MixedGCCountTarget=4:** Default is 8. Because we are aiming to collect slower, with less old
gen usage, try to reclaim old gen memory faster to avoid running out of old.

Doing this greatly reduces pause times in young collections as copying data up to 15 times in
survivor space for a tenured object really takes a lot of time for actually old memory. Ideally
the GC engine would track average age for objects instead and tenure out data faster, but that
is not how it works.
10. **G1MixedGCLiveThresholdPercent:** Controls when to include regions in mixed GCs in the young
GC collection, keeping old gen tidy without doing a normal old gen GC collection. When your
memory is less than this percent, old gen won't even be included in 'mixed' collections. Mixed
are not as heavy as a full old collection, so having small incremental cleanups of old keeps
memory usage light.

Considering average GC rate is 10s to the upwards of minutes per young collection, this does not
result in any 'garbage' being promoted, and just delays longer lived memory to be collected in
mixed GCs.
Default is 65 to 85 depending on the Java version, we are setting that to 90 to ensure we reclaim garbage
in old gen as fast as possible to retain as much free regions as we can.

7. **SurvivorRatio=32:** Because we drastically reduced MaxTenuringThreshold, we will be reducing
use of survivor space drastically. This frees up more regions to be used by eden instead.
12. **G1ReservePercent=20:** Minecraft memory allocation rate in up-to-date versions is really insane. We
run the risk of a dreaded "to-space exhaustion" not having enough memory free to move data
around. This ensures more memory is waiting to be used for this operation. Default is 10, so we
are giving another 10 to it.

8. **AlwaysPreTouch:** AlwaysPreTouch gets the memory setup and reserved at process start ensuring
it is contiguous, improving the efficiency of it more. This improves the operating systems
memory access speed. Mandatory to use Transparent Huge Pages

9. **+DisableExplicitGC:** Many plugins think they know how to control memory, and try to invoke
garbage collection. Plugins that do this trigger a full garbage collection, triggering a massive
lag spike. This flag disables plugins from trying to do this, protecting you from their bad
code.
13. **G1RSetUpdatingPauseTimePercent=5:** Default is 10% of time spent during pause updating RSets,
reduce this to 5% to make more of it concurrent to reduce pause durations.

10. **MaxGCPauseMillis=200:** This setting controls how much memory is used in between the minimum
and maximum ranges specified for your new generation. This is a "goal" for how long you want
your server to pause for collections. 200 is aiming for at most loss of 4 ticks. This will
result in a short TPS drop, however the server can make up for this drop instantly, meaning it
will have no meaningful impact on your TPS. 200ms is lower than players can recognize. In
testing, having this value constrained to an even lower number results in G1 not recollecting
memory fast enough and potentially running out of old gen triggering a full collection. Just
because this number is 200 does not mean every collection will be 200. It means it can use up to
200 if it really needs it, and we need to let it do its job when there is memory to collect.
14. **MaxTenuringThreshold=1:** Minecraft has a really high allocation rate of memory. Of that
memory, most is reclaimed in the eden generation. However, transient data will overflow into
survivor. Initially played with completely removing survivor and had decent results, but does
result in transient data making its way to old which is not good. Max Tenuring 1 ensures that we
do not promote transient data to old generation, but anything that survives 2 passes of GC is
just going to be assumed as longer-lived.

11. **+ParallelRefProcEnabled:** Optimizes the GC process to use multiple threads for weak reference
checking. Not sure why this isn't default...
Doing this greatly reduces pause times in young collections as copying data up to 15 times in
survivor space for a tenured object really takes a lot of time for actually old memory. Ideally
the GC engine would track average age for objects instead and tenure out data faster, but that
is not how it works.

12. **G1RSetUpdatingPauseTimePercent=5:** Default is 10% of time spent during pause updating RSets,
reduce this to 5% to make more of it concurrent to reduce pause durations.
Considering average GC rate is 10s to the upwards of minutes per young collection, this does not
result in any 'garbage' being promoted, and just delays longer lived memory to be collected in
mixed GCs.

13. **G1MixedGCCountTarget=4:** Default is 8. Because we are aiming to collect slower, with less old
gen usage, try to reclaim old gen memory faster to avoid running out of old.
15. **SurvivorRatio=32:** Because we drastically reduced MaxTenuringThreshold, we will be reducing
use of survivor space drastically. This frees up more regions to be used by eden instead.

14. **G1HeapRegionSize=8M+:** Default is auto calculated. Super important for Minecraft, especially
1.15, as with low memory situations, the default calculation will in most times be too low. Any
memory allocation half of this size (4MB) will be treated as "Humongous" and promote straight to
old generation and is harder to free. If you allow Java to use the default, you will be
destroyed with a significant chunk of your memory getting treated as Humongous.
16. **UseCompactObjectHeaders** Shrinks object headers, doing so saves memory and cpu usage.

15. **+PerfDisableSharedMem:** Causes GC to write to file system which can cause major latency if
disk IO is high - see https://www.evanjones.ca/jvm-mmap-pause.html

### Transparent huge pages

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