Why does the JVM have an -Xms option? Why do we care about a minimum heap size? Why not just 0? It's super easy to allocate RAM, so I don't see the point of forcing a minimum heap size.
In my searching, I see that it's customary to set -Xms (minimum heap size) and -Xmx (maximum heap size) to the same value.
I am having a hard time finding a clear and rational basis for this custom or why -Xms even exists. Rather, I find a lot of communal reinforcement. On occasion, I see it justified by a flaky theory, such that the JVM is unusually slow at allocating additional RAM as it grows the heap size.
While this came up as I was optimizing Solr, it seems that fussing with the heap size is a common consideration with JVMs.
As a curious data point, you'll see two memory-usage dips here:
Before dip 1: -Xms14g -Xmx14g
Between dip 1 and 2: -Xms0g -Xmx14g
After dip 2: -Xmx14g
After dip 2, Solr reported to me that it was only using a couple hundred MBs of heap space even though the JVM gobbled up many GBs of RAM.
In case it matters, I am on the current release of OpenJDK.
To summarize, is there a rational and fact-based basis for:
Setting -Xms to something other than 0.
The custom of setting -Xms and -Xmx to the same value.
Why -Xms even exists.
I think the fact-based basis will help with a more informed basis for managing heap-size options.
You have three questions.
First question:
Is there a rational and fact-based basis for setting -Xms to something other than 0?
Looking as far back as version 8, we can see (among other things) the following for -Xms:
Sets the minimum and the initial size (in bytes) of the heap.
So, setting -Xms to 0 would mean setting the heap to 0. I'm not sure what your expectation would be, but.. the JVM needs at least some amount of heap to do things (like run a garbage collector).
Second question:
Is there a rational and fact-based basis for the custom of setting -Xms and -Xmx to the same value?
Yes, if you expect to use a certain amount of memory over time, but not necessarily right away. You could allocate the full amount of memory up front so that any allocation costs are out of the way.
For example, consider an app that launches needing less than 1GB of memory, but over time it grows (normally, correctly) to 4GB. You could run that app with -Xms1g -Xmx4g – so, start with 1GB and do periodic allocations over time to reach 4GB. Or you could run with -Xms4g -Xmx4g – tell the JVM to allocate 4GB now, even if it's not going to be used right away.
Allocating memory from an underlying operating system has a cost, and might be expensive enough that you'd like to do that early in the application life, instead of some later point where that cost might be more impactful.
Third question:
Is there a rational and fact-based basis for why -Xms even exists?
Yes, it allows tuning JVM behavior. Some applications don't need to do this, but others do. It's useful to be able to set values for lower, upper, or both (lower and upper together). Way beyond this, there's a whole world of garbage collector tuning, too.
A little more detail on how -Xms is used (below) could give you some initial garbage collection topics to read about (old generation, young generation):
If you do not set this option, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation.
Related
Edit:
I'm changing my question to make it more clarify.
Here is my app, running with defualt gc in java8:
Yes, there are a lot of gc time, but, my commited memory fits to my actual used memory (I know this is not the desired behabiour)
Now, lets take a look what happens when we set up G1 gc:
You can see the commited memory is much much larger than the used one.
You might be wondering what has changed between first and second execution:
The first one goes with JVM_ARGS: -Xms1024m -Xmx20048m -XX:MaxGCPauseMillis=100
The second one:-XX:+UseG1GC -Xms1024m -Xmx20048m -XX:MaxGCPauseMillis=100 -XX:+UseStringDeduplication
I've been reading a lot about G1 GC, but, I can't understand why this simple change, makes my memory behabes so diferent?
As described in this document from Oracle -Xmx sets the maximum available Memory for the jvm and -Xms the initial memory, so with your values, the jvm uses 1024mb at stat up and up to 4096m when running. So this setting is more a limit then a consumption value.
Compare it to a bucket: If it can hold 10 Liters, you don't have to fill 10 Liters into this bucket, you could also fill it with only 2 Liters
the Heap size of my java application is continuously growing till it reaches the Max Heap sizeof 1G. Why is that so?
I start my application with those parameters:
java -Xmx1G -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahUncommitDelay=5000 -XX:ShenandoahGuaranteedGCInterval=10000 -Dlog4j.configurationFile=./log4j2.xml -jar applicaiton.jar
Edit:
When I restart my application you see there isn't a gap between "Heap size" and "Used Heap" but this gap is getting bigger and bigger, can I somehow limit that gap?
I am not particularly familiar with the way that the new Shenandoah GC normally behaves. However, there is nothing particularly alarming (to me) with that graph.
According to https://shipilev.net/talks/devoxx-Nov2017-shenandoah.pdf, the modus operandi (MO) of a this GC with regards to memory utilization is a bit different to some other collectors.
"We shall take all the memory when we need it, but we shall also give it back when we don’t".
If there is significant load on the allocator (i.e. lots of objects being allocated) Shenandoah will aggressively expand the heap. This is based on the observation that a low-pause GC is most efficient (and most likely to keep up!) if it has plenty of space to work in.
But the flipside is that if your system is idle, the GC will give memory back to the OS more freely than many other GCs.
This seems to fit the memory graph in your question.
The other thing to note is that the heap size (orange) is nowhere near your max-heap limit. If you get close to that limit, the GC will stop growing the heap.
Finally, note that you can apparently encourage Shenandoah to give back uncommitted memory faster by using a smaller value for the -XX:ShenandoahUncommitDelay=<millis> option. However, it is recommended to NOT make it too small because that is liable to slow down the allocator.
(Source: https://www.javacodegeeks.com/2017/11/minimize-java-memory-usage-right-garbage-collector.html)
Reference: https://bugs.java.com/bugdatabase/view_bug.do?bug_id=8186315
I'm really struggling to find out what MinRAMPercentage does, especially compared to InitialRAMPercentage.
I assumed that InitialRAMPercentage sets the amount of heap at startup, that MinRAMPercentage and MaxRAMPercentage set the bottom and top limit of heap that the JVM is allowed to shrink/grow to.
Apparently that is not the case. When I start a JVM (with UseContainerSupport, having these new memory setting parameters) like so:
java -XX:+UseContainerSupport -XX:InitialRAMPercentage=40.0 -XX:MinRAMPercentage=20.0 -XX:MaxRAMPercentage=80.0 -XX:+PrintFlagsFinal -version | grep Heap
InitialHeap and MaxHeap get set, there is no "Minimum Heap Size" value that I can find; Consequently, that MinRAMPercentage never seems to get used.
Super confused, and apparently, I'm not the only one; the OpenJ9 dudes seem to also not fully parse the intent of these options, as I've gathered here and here. They seem to have opted to simply not implement MinRAMPercentage afaics.
So: What is the real intended usage and effect of setting MinRAMPercentage?
-XX:InitialRAMPercentage is used to calculate initial heap size when InitialHeapSize / -Xms is not set.
It sounds counterintuitive, but both -XX:MaxRAMPercentage and -XX:MinRAMPercentage are used to calculate maximum heap size when MaxHeapSize / -Xmx is not set:
For systems with small physical memory MaxHeapSize is estimated as
phys_mem * MinRAMPercentage / 100 (if this value is less than 96M)
Otherwise (non-small physical memory) MaxHeapSize is estimated as
MAX(phys_mem * MaxRAMPercentage / 100, 96M)
The exact formula is a bit more complicated as it also takes other factors into account.
Note: the algorithm for calculating initial and maximum heap size depends on the particular JVM version. The preferred way to control the heap size is to set Xmx and Xms explicitly.
See also this question.
Depends on your container memory also, So lets suppose you have container memory as 1 GB then in this case -XX:MaxRAMPercentage=80 will be used to determine the max heap ~ 800mb heap memory will be used
And suppose you have container memory less than 250mb then -XX:MinRAMPercentage=20.0 will be used ~ 50mb heap memory will be used
use this article to understand more XX:MinRAMPercentage,XX:MaxRAMPercentage
I have always given an assumed heap size to my application and while the app is running, I monitor and modify / tune the heap size.
Is there a way in which I can calculate the initial heap required more or less accurately.
For the best performance in a Java EE style environment, i.e. when the application is meant to be running for very long periods of time (months or weeks), then it is best to set the minimum and maximum heap size to be the same.
The best way to size the heap in this case is to gather data on how your application runs over time. With a weeks worth of verbose GC log, we can import that data into GCViewer. Looking at the troughs of the graph, we can take an average and see the minimum retained set after each garbage collection. This is the amount of data, on average, kept in the heap for normal running. So any heap size should be at least that level. Since we're setting minimum and maximum to the same here, we need to add more space to compensate for spikes. How much to add depends on your use case, but somewhere between 25-35% is a start.
With this method, always remember to keep monitoring your heap and GC behaviour using verbose GC (which is recommended by Oracle to run even in production).
Important: This is assuming that your application is an always-on type of application. If you're considering a desktop java application, then the behaviour will be very very different and this should not be seen as a reliable method in that case. As #juhist said in his/her answer, just the maximum should be set and the JVM will handle the rest.
Why do you monitor and modify / tune the heap size?
Java has been designed in a way that the heap automatically grows to the needed size as long as the maximum heap size is large enough. You shouldn't modify the heap size manually at all.
I would rely on the automatic Java algorithms for adjusting the heap size and just specify the maximum heap size at program startup. You should take into account how many java processes you're running in parallel when deciding the maximum heap size.
Currently in our testing environment the max and min JVM heap size are set to the same value, basically as much as the dedicated server machine will allow for our application. Is this the best configuration for performance or would giving the JVM a range be better?
Peter 's answer is correct in that -Xms is allocated at startup and it will grow up to -Xmx (max heap size) but it's a little misleading in how he has worded his answer. (Sorry Peter I know you know this stuff cold).
Setting ms == mx effectively turns off this behavior. While this used to be a good idea in older JVMs, it is no longer the case. Growing and shrinking the heap allows the JVM to adapt to increases in pressure on memory yet reduce pause time by shrinking the heap when memory pressure is reduced. Sometimes this behavior doesn't give you the performance benefits you'd expect and in those cases it's best to set mx == ms.
OOME is thrown when heap is more than 98% of time is spent collecting and the collections cannot recover more than 2% of that. If you are not at max heaps size then the JVM will simply grow so that you're beyond that boundaries. You cannot have an OutOfMemoryError on startup unless your heap hits the max heap size and meets the other conditions that define an OutOfMemoryError.
For the comments that have come in since I posted. I don't know what the JMonitor blog entry is showing but this is from the PSYoung collector.
size_t desired_size = MAX2(MIN2(eden_plus_survivors, gen_size_limit()),
min_gen_size());
I could do more digging about but I'd bet I'd find code that serves the same purpose in the ParNew and PSOldGen and CMS Tenured implementations. In fact it's unlikely that CMS would be able to return memory unless there has been a Concurrent Mode Failure. In the case of a CMF the serial collector will run and that should include a compaction after which top of heap would most likely be clean and therefore eligible to be deallocated.
Main reason to set the -Xms is for if you need a certain heap on start up. (Prevents OutOfMemoryErrors from happening on start up.) As mentioned above, if you need the startup heap to match the max heap is when you would match it. Otherwise you don't really need it. Just asks the application to take up more memory that it may ultimately need. Watching your memory use over time (profiling) while load testing and using your application should give you a good feel for what to need to set them to. But it isn't the worse thing to set them to the same on start up. For a lot of our apps, I actually start out with something like 128, 256, or 512 for min (startup) and one gigabyte for max (this is for non application server applications).
Just found this question on stack overflow which may also be helpful side-effect-for-increasing-maxpermsize-and-max-heap-size. Worth the look.
AFAIK, setting both to the same size does away with the additional step of heap resizing which might be in your favour if you pretty much know how much heap you are going to use. Also, having a large heap size reduces GC invocations to the point that it happens very few times. In my current project (risk analysis of trades), our risk engines have both Xmx and Xms to the same value which pretty large (around 8Gib). This ensures that even after an entire day of invoking the engines, almost no GC takes place.
Also, I found an interesting discussion here.
Definitely yes for a server app. What's the point of having so much memory but not using it?
(No it doesn't save electricity if you don't use a memory cell)
JVM loves memory. For a given app, the more memory JVM has, the less GC it performs. The best part is more objects will die young and less will tenure.
Especially during a server startup, the load is even higher than normal. It's brain dead to give server a small memory to work with at this stage.
From what I see here at http://java-monitor.com/forum/showthread.php?t=427
the JVM under test begins with the Xms setting, but WILL deallocate memory it doesn't need and it will take it upto the Xmx mark when it needs it.
Unless you need a chunk of memory dedicated for a big memory consumer initially, there's not much of a point in putting in a high Xms=Xmx. Looks like deallocation and allocation occur even with Xms=Xmx