Tuning the JVM – G1GC Garbage Collector Flags for Minecraft
After many weeks of studying the JVM, Flags, and testing various combinations, I came up with a highly tuned set of Garbage Collection flags for Minecraft. I tested these on my server, and have been used for years. I then announced my research to the public, and to this day, many servers have been using my flag recommendations for years and reporting great improvement to garbage collection behavior.
These flags are the result of a ton of effort, and results of seeing it in production on various server sizes, plugin lists and server types. They have proven themselves repeatedly.
I strongly suggest using these flags to start your server. These flags help keep your server running CONSISTENT without any large garbage collection spikes. CPU may be slightly higher, but your server will be overall more reliable and stable TPS.
If these flags help your server, consider donating!
The JVM Startup Flags to use – MC 1.15 (Java 8+, MC 1.8+) Update
Use these flags exactly, only changing Xmx and Xms. These flags work and scale accordingly to any size of memory, even 500MB but 1.15 will not do well with such low memory…)
IMPORTANT – READ – Don’t use ALL of your memory!! PTERODACTYL USERS!
I recommend using at least 6-10GB, No matter how few players! If you can’t afford 10GB of memory, give as much as you can, but ensure you leave the operating system some memory too. G1GC operates better with more memory.
If you are running with 12GB or less memory for MC, you should not adjust these parameters.
If you are using an Xmx value greater than 12G
If you have and use more than 12GB of memory, adjust the following:
NOTICE: If you see increase in old generation collections after this, revert back to the base flags!
Explanation of these changes:
- Base flag set aims for 30/40 to reduce risk of to space issues. With more memory, less of an issue. We can give more to new generation with 40/50, as well as reduce reserve percent since the default reserve will already be larger.
- Region Size increase helps reduce humongous allocations, and speeds up remarking. We need a smaller region size at smaller heaps to ensure an adequate amount of regions available
- We can start looking for old generation memory to reclaim with more of a delay with IHOP at 20 since we have more old generation available to space on CPU.
Java GC Logging
Are you having old gen issues with these flags? Help me help you! Add the following flags based on your java version to enable GC Logging:
-Xloggc:gc.log -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=5 -XX:GCLogFileSize=1M
Once you start seeing old generation collections in Timings, grab the logs/gc.log file (same location as your latest.log) and send it to me on Paper Discord to analyze.
GC logging does not hurt your performance and can be left on at all times. The files will not take up much space (5MB)
Technical Explanation of the Flags:
- -Xms matching -Xmx – Why: You should never run your server with the case that -Xmx can run the system completely out of memory. Your server should always be expected to use the entire -Xmx!
You should then ensure the OS has extra memory on top of that Xmx for non MC/OS level things. Therefore, you should never run MC with -Xmx settings you can’t support if java uses it all.Now, that means if -Xms is lower than -Xmx -YOU HAVE UNUSED MEMORY!Unused memory is wasted memory.G1 (and probably even CMS to a certain threshold, but I’m only stating what I’m sure about) operates better with the more memory it’s given. G1 adaptively chooses how much memory to give to each region to optimize pause time. If you have more memory than it needs to reach an optimal pause time, G1 will simply push that extra into the old generation and it will not hurt you (This may not be the case for CMS, but is the case for G1).The fundamental idea of improving GC behavior is to ensure short lived objects die young and never get promoted. With the more memory G1 has, the better assurance you will get that objects are not getting prematurely promoted to the old generation.G1 Operates differently than previous collectors and is able to handle larger heaps more efficiently.
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!
- UnlockExperimentalVMOptions – needed for some the below options
- G1NewSizePercent: These are the important ones. In CMS and other Generations, tweaking the New Generation results in FIXED SIZE New Gen and usually is done through explicit size setting with -Xmn.With G1, things are better! 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 a memory allocation rate, ranging to at least 800 Megabytes a second on a 30 player server! And this is mostly short lived objects (Block Position)
Now, this means MC REALLY needs more focus on New Generation to be able to even support this allocation rate. If your new gen is too small, you will be running new gen collections 1-2+ times per second, which is really bad.You will have so many pauses that TPS has risk of suffering, and the server will not be able to keep up with the cost of GC’s.Then combine the fact that objects will now promote faster, resulting in your Old Gen growing faster. Given more NewGen, 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.
- G1MixedGCLiveThresholdPercent: Controls when to include regions in Mixed GC’s 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 Java Version, we are setting to 90 to ensure we reclaim garbage in old gen as fast as possible to retain as much free regions as we can.My Old flag set had this at 35 which was a bug. I had the intent of this flag inverted, as I thought 35 was what 65 does. You should not be using 35 for this number.
- G1ReservePercent=20: MC 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.
- 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 Garbage Collection is just going to be assumed as longer-lived.
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.
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 GC’s.
- 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.
- 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
- +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.
- 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 to 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.
- +ParallelRefProcEnabled: Optimizes the GC process to use multiple threads for weak reference checking. Not sure why this isn’t default….
- 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.
- 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.
- 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.
- +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
Using Large Pages
Also for Large Pages – It’s even more important to use -Xms = -Xmx! Large Pages needs to have all of the memory specified for it or you could end up without the gains. This memory will not be used by the OS anyways, so use it.
Additionally use these flags (Metaspace is Java 8 Only, don’t use it for Java7):
Transparent Huge Pages
Controversial Feature but may be usable if you can not configure your host for real HugeTLBFS. try adding -XX:+UseTransparentHugePages but it’s extremely important you also have AlwaysPreTouch set. Otherwise THP will likely hurt you. I have not measured how THP works for MC or its impact with AlwaysPreTouch, so this section is for the advanced users who want to experiement.
Thanks to https://product.hubspot.com/blog/g1gc-fundamentals-lessons-from-taming-garbage-collection for helping reinforce my understanding of the flags and introduce improvements!
- 5/2/2020: Added +PerfDisableSharedMem, Adjusted MixedGCTarget to 4
- 4/25/2020: Removed OmitStackTraces since it could cause performance issues with some plugins (but not everyone)
- 4/5/2020: Massive refactor of the flag suggestions. Takes a new approach at optimizing pause times. Flags may still be changing. These changes are mandatory for MC 1.15
- 10/4/2018: Removed AggressiveOpts and InitiatingHeapOccupancyPercent. Aggressive is removed in Java 11, and IHOP may hurt performance in Java 11. You should remove them for Java 8 too.
- 8/18/2018: Adjusted MixedGCLiveThreshold to 35 (from 50) to ensure mixed GC’s start earlier.
Added notes about recommended use of 10GB of memory.
Added more flag documentation
- 5/24/2018: Added -XX:+ParallelRefProcEnabled
Advanced Minecraft Marketing Analytics
I’ve mentioned a few times the power of my analytics system in the Spigot community and recently in AdminCraft on Reddit. Most advertising campaigns simply track impressions and clicks. But to a server owner, we need more data than that…. We need to know who actually joined the server and continues to play!
I have designed such a system, where I can track clicks, how many joined the game server, then out of those, who stay to the day 5, 15, 30, 60 and 90 markers! I can see when campaigns generate a lot of clicks, yet low players, or worse: players who don’t actually stay.
This article will give a rough idea on how I did it incase someone is curious to implement it on their own.
I want to be straight that the implementation isn’t perfect or the most optimized, but hey it gives you data where you normally would not have data, so that’s a clear winner!
Requirements / Disclaimer
This system only works for advertising campaigns that involve website clicks. Campaigns that serve purely as a banner and a server connect address can not be realistically tracked.
My implementation all revolves around Google Analytics, but the concept can work for any form of marketing system you want to adapt it to.
You should be using analytics.js (I do not think this works with ga.js, the old Analytics system) on every page of your site.
You will also need to be an experienced developer or have one on hand.
I WILL NOT HELP ANYONE IMPLEMENT THIS. I do not do contract work / consulting services, but feel free to ask questions on Spigot IRC.
Tracking that click
First off, my system requires that all inbound campaigns need to land the user on the website. If someone see’s your server connect address and connects directly, then you can’t track that – but hey! That’s usually a FREE conversion , so don’t complain 🙂
Here is an example database table that I use:
CREATE TABLE `conversions` ( `clientid` varchar(255) CHARACTER SET latin1 NOT NULL, `convert_id` varchar(255) CHARACTER SET latin1 NOT NULL, `ip` int(10) unsigned NOT NULL, `ua` varchar(255) CHARACTER SET latin1 DEFAULT NULL, `date` int(11) unsigned NOT NULL, `lp` text CHARACTER SET latin1, `converted` tinyint(4) unsigned NOT NULL DEFAULT '0' PRIMARY KEY (`clientid`), KEY `ip` (`ip`), KEY `converted` (`converted`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
our goal is to assign a Google Analytics Client Id to every user. Additionally, I assign a unique ID to every user separate to the client ID, as some users may have addons that constantly change their Google Analytics Client ID, so this prevents inserting an entry into DB every time their client id changes.
When a user comes in and has a Google Client ID set, assign them a convert ID. then insert an entry into the database.
Explanation of fields:
- clientid: Google Analytics Client ID
- convert_id: Field you generate to associate to a session
- ip: users IP address – to be passed later to Google Analytics for conversion and match to a newly joined player
- ua: Users User Agent – to be passed later to Google Analytics for conversion
- date: current time when seen
- lp: URL the player first landed on – so you can know what web page they visited.
- converted: has this record triggered a conversion not.
So, once you have their IP and GA client ID tagged into the database off that first page load… You got them 🙂 If they join, you will know what campaign they came from.
Now that you have them in the database, you need to have a monitoring system that checks your game server. This is the trickiest part of pulling this off, as how you do this will really vary based on your servers data infrastructure.
We have a users database table that we can easily query on and find who has played in the past few hours and has not triggered conversion events. So we run a query, get the users IP address, and then look for matching conversion table entries on the same IP address that is also not marked as converted.
In the event 2 people joined on same IP from 2 diff devices, you would have 2 entries in the conversions table, and then each would match up to each other, still resulting in 2 conversions. If 2 accounts join but only 1 device hit the website, then only 1 conversion should be marked initially, but they are likely to then trigger a 2nd conversion event once they hit the website with a new device (that doesn’t match the convert_id you generated).
Now… the actual marking of a conversion when this happens!
Setting up your conversion event
Now you need to configure Google Analytics to have a goal to count the conversions.
Go to Admin > Your Property > Your Primary View > Goals
Add a new Goal using an Event like so:
This step doesn’t have to be done first, but it does need to be done to view it and there’s no benefit to do it after rather than before.
Google Analytics Measurement Protocol
The way the Google Analytics tracking system works is extensively documented, and even encouraged for you to do your own backend server side reporting of the event on behalf of the user!
Read all the documentation: Google Analytics Measurement Protocol.
Now we’ve got a list of rows from the conversion table of who joined the game server, with their GA Client Id IP and URL.
Now your backend monitoring system needs to call out to Google Analytics in the exact same fashion the client browsers do, but I’ll save you the trouble, here is the code I use to do this:
I do even further events like Day 5/15/30/60/90 tracking, so I pass in other events for those. To support that type of data, you will have to analyze your data and figure out the best way to compute who should be converted for those cases.
Tagging Every Inbound Link
Now you have the infrastructure set up, you need to ensure every direct campaign you control is tagged! This involves setting utm_* parameters on your inbound links.
Learn more about UTM Parameters
I personally use our emc.gs url shortner, then create links that redirect to pages with the utm parameters, so that I can do “reddit.emc.gs” and it will tag that link with the Reddit tracking parameters. If you want to set up a url shortener like we have, check out YourLS.
I have modified it for our needs though, making it so &gac = &utm_campaign, &gas = &utm_source and &gam = &utm_medium for quick changes of properties like reddit.emc.gs/?gam=ad-4
I suggest setting up a redirect system on your main website domain and not an alternate domain like we do, because many advertising systems dislike domain mismatches, and we had to set up empireminecraft.com/go/foo to redirect to emc.gs/foo which then redirects back to the target of emc.gs/foo just to trick some of those platforms.
Enjoy having data about your campaigns and who sends you the best traffic 🙂0