Java using much more memory than heap size (or size correctly Docker memory limit)

java using more memory than xmx
java 11 default heap size
java.lang.outofmemoryerror: java heap space kubernetes
docker jvm heap size
docker in java
xx:maxram
invalid initial heap size docker
docker jvm arguments

For my application, the memory used by the Java process is much more than the heap size.

The system where the containers are running starts to have memory problem because the container is taking much more memory than the heap size.

The heap size is set to 128 MB (-Xmx128m -Xms128m) while the container takes up to 1GB of memory. Under normal condition, it needs 500MB. If the docker container has a limit below (e.g. mem_limit=mem_limit=400MB) the process gets killed by the out of memory killer of the OS.

Could you explain why the Java process is using much more memory than the heap? How to size correctly the Docker memory limit? Is there a way to reduce the off-heap memory footprint of the Java process?


I gather some details about the issue using command from Native memory tracking in JVM.

From the host system, I get the memory used by the container.

$ docker stats --no-stream 9afcb62a26c8
CONTAINER ID        NAME                                                                                        CPU %               MEM USAGE / LIMIT   MEM %               NET I/O             BLOCK I/O           PIDS
9afcb62a26c8        xx-xxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.0acbb46bb6fe3ae1b1c99aff3a6073bb7b7ecf85   0.93%               461MiB / 9.744GiB   4.62%               286MB / 7.92MB      157MB / 2.66GB      57

From inside the container, I get the memory used by the process.

$ ps -p 71 -o pcpu,rss,size,vsize
%CPU   RSS  SIZE    VSZ
11.2 486040 580860 3814600

$ jcmd 71 VM.native_memory
71:

Native Memory Tracking:

Total: reserved=1631932KB, committed=367400KB
-                 Java Heap (reserved=131072KB, committed=131072KB)
                            (mmap: reserved=131072KB, committed=131072KB) 

-                     Class (reserved=1120142KB, committed=79830KB)
                            (classes #15267)
                            (  instance classes #14230, array classes #1037)
                            (malloc=1934KB #32977) 
                            (mmap: reserved=1118208KB, committed=77896KB) 
                            (  Metadata:   )
                            (    reserved=69632KB, committed=68272KB)
                            (    used=66725KB)
                            (    free=1547KB)
                            (    waste=0KB =0.00%)
                            (  Class space:)
                            (    reserved=1048576KB, committed=9624KB)
                            (    used=8939KB)
                            (    free=685KB)
                            (    waste=0KB =0.00%)

-                    Thread (reserved=24786KB, committed=5294KB)
                            (thread #56)
                            (stack: reserved=24500KB, committed=5008KB)
                            (malloc=198KB #293) 
                            (arena=88KB #110)

-                      Code (reserved=250635KB, committed=45907KB)
                            (malloc=2947KB #13459) 
                            (mmap: reserved=247688KB, committed=42960KB) 

-                        GC (reserved=48091KB, committed=48091KB)
                            (malloc=10439KB #18634) 
                            (mmap: reserved=37652KB, committed=37652KB) 

-                  Compiler (reserved=358KB, committed=358KB)
                            (malloc=249KB #1450) 
                            (arena=109KB #5)

-                  Internal (reserved=1165KB, committed=1165KB)
                            (malloc=1125KB #3363) 
                            (mmap: reserved=40KB, committed=40KB) 

-                     Other (reserved=16696KB, committed=16696KB)
                            (malloc=16696KB #35) 

-                    Symbol (reserved=15277KB, committed=15277KB)
                            (malloc=13543KB #180850) 
                            (arena=1734KB #1)

-    Native Memory Tracking (reserved=4436KB, committed=4436KB)
                            (malloc=378KB #5359) 
                            (tracking overhead=4058KB)

-        Shared class space (reserved=17144KB, committed=17144KB)
                            (mmap: reserved=17144KB, committed=17144KB) 

-               Arena Chunk (reserved=1850KB, committed=1850KB)
                            (malloc=1850KB) 

-                   Logging (reserved=4KB, committed=4KB)
                            (malloc=4KB #179) 

-                 Arguments (reserved=19KB, committed=19KB)
                            (malloc=19KB #512) 

-                    Module (reserved=258KB, committed=258KB)
                            (malloc=258KB #2356) 

$ cat /proc/71/smaps | grep Rss | cut -d: -f2 | tr -d " " | cut -f1 -dk | sort -n | awk '{ sum += $1 } END { print sum }'
491080

The application is a web server using Jetty/Jersey/CDI bundled inside a fat far of 36 MB.

The following version of OS and Java are used (inside the container). The Docker image is based on openjdk:11-jre-slim.

$ java -version
openjdk version "11" 2018-09-25
OpenJDK Runtime Environment (build 11+28-Debian-1)
OpenJDK 64-Bit Server VM (build 11+28-Debian-1, mixed mode, sharing)
$ uname -a
Linux service1 4.9.125-linuxkit #1 SMP Fri Sep 7 08:20:28 UTC 2018 x86_64 GNU/Linux

https://gist.github.com/prasanthj/48e7063cac88eb396bc9961fb3149b58


Virtual memory used by a Java process extends far beyond just Java Heap. You know, JVM includes many subsytems: Garbage Collector, Class Loading, JIT compilers etc., and all these subsystems require certain amount of RAM to function.

JVM is not the only consumer of RAM. Native libraries (including standard Java Class Library) may also allocate native memory. And this won't be even visible to Native Memory Tracking. Java application itself can also use off-heap memory by means of direct ByteBuffers.

So what takes memory in a Java process?

JVM parts (mostly shown by Native Memory Tracking)
  1. Java Heap

    The most obvious part. This is where Java objects live. Heap takes up to -Xmx amount of memory.

  2. Garbage Collector

    GC structures and algorithms require additional memory for heap management. These structures are Mark Bitmap, Mark Stack (for traversing object graph), Remembered Sets (for recording inter-region references) and others. Some of them are directly tunable, e.g. -XX:MarkStackSizeMax, others depend on heap layout, e.g. the larger are G1 regions (-XX:G1HeapRegionSize), the smaller are remembered sets.

    GC memory overhead varies between GC algorithms. -XX:+UseSerialGC and -XX:+UseShenandoahGC have the smallest overhead. G1 or CMS may easily use around 10% of total heap size.

  3. Code Cache

    Contains dynamically generated code: JIT-compiled methods, interpreter and run-time stubs. Its size is limited by -XX:ReservedCodeCacheSize (240M by default). Turn off -XX:-TieredCompilation to reduce the amount of compiled code and thus the Code Cache usage.

  4. Compiler

    JIT compiler itself also requires memory to do its job. This can be reduced again by switching off Tiered Compilation or by reducing the number of compiler threads: -XX:CICompilerCount.

  5. Class loading

    Class metadata (method bytecodes, symbols, constant pools, annotations etc.) is stored in off-heap area called Metaspace. The more classes are loaded - the more metaspace is used. Total usage can be limited by -XX:MaxMetaspaceSize (unlimited by default) and -XX:CompressedClassSpaceSize (1G by default).

  6. Symbol tables

    Two main hashtables of the JVM: the Symbol table contains names, signatures, identifiers etc. and the String table contains references to interned strings. If Native Memory Tracking indicates significant memory usage by a String table, it probably means the application excessively calls String.intern.

  7. Threads

    Thread stacks are also responsible for taking RAM. The stack size is controlled by -Xss. The default is 1M per thread, but fortunately the things are not so bad. OS allocates memory pages lazily, i.e. on the first use, so the actual memory usage will be much lower (typically 80-200 KB per thread stack). I wrote a script to estimate how much of RSS belongs to Java thread stacks.

    There are other JVM parts that allocate native memory, but they do not usually play a big role in total memory consumption.

Direct buffers

An application may explicitly request off-heap memory by calling ByteBuffer.allocateDirect. The default off-heap limit is equal to -Xmx, but it can be overridden with -XX:MaxDirectMemorySize. Direct ByteBuffers are included in Other section of NMT output (or Internal before JDK 11).

The amount of used direct memory is visible through JMX, e.g. in JConsole or Java Mission Control:

Besides direct ByteBuffers there can be MappedByteBuffers - the files mapped to virtual memory of a process. NMT does not track them, however, MappedByteBuffers can also take physical memory. And there is no a simple way to limit how much they can take. You can just see the actual usage by looking at process memory map: pmap -x <pid>

Address           Kbytes    RSS    Dirty Mode  Mapping
...
00007f2b3e557000   39592   32956       0 r--s- some-file-17405-Index.db
00007f2b40c01000   39600   33092       0 r--s- some-file-17404-Index.db
                           ^^^^^               ^^^^^^^^^^^^^^^^^^^^^^^^
Native libraries

JNI code loaded by System.loadLibrary can allocate as much off-heap memory as it wants with no control from JVM side. This also concerns standard Java Class Library. In particular, unclosed Java resources may become a source of native memory leak. Typical examples are ZipInputStream or DirectoryStream.

JVMTI agents, in particular, jdwp debugging agent - can also cause excessive memory consumption.

This answer describes how to profile native memory allocations with async-profiler.

Allocator issues

A process typically requests native memory either directly from OS (by mmap system call) or by using malloc - standard libc allocator. In turn, malloc requests big chunks of memory from OS using mmap, and then manages these chunks according to its own allocation algorithm. The problem is - this algorithm can lead to fragmentation and excessive virtual memory usage.

jemalloc, an alternative allocator, often appears smarter than regular libc malloc, so switching to jemalloc may result in a smaller footprint for free.

Conclusion

There is no guaranteed way to estimate full memory usage of a Java process, because there are too many factors to consider.

Total memory = Heap + Code Cache + Metaspace + Symbol tables +
               Other JVM structures + Thread stacks +
               Direct buffers + Mapped files +
               Native Libraries + Malloc overhead + ...

It is possible to shrink or limit certain memory areas (like Code Cache) by JVM flags, but many others are out of JVM control at all.

One possible approach to setting Docker limits would be to watch the actual memory usage in a "normal" state of the process. There are tools and techniques for investigating issues with Java memory consumption: Native Memory Tracking, pmap, jemalloc, async-profiler.

Update

Here is a recording of my presentation Memory Footprint of a Java Process.

In this video, I discuss what may consume memory in a Java process, how to monitor and restrain the size of certain memory areas, and how to profile native memory leaks in a Java application.

Troubleshooting Memory Issues in Java Applications, . The JVM default without this option is 1MB. Be mindful of heavyweight monitoring agents. The limit-heap-and-container container is a bit over-provisioned on memory. However, this exercise demonstrated that the JVM needed significantly more memory (20%) that what is implied by its heap settings, even for nearly the simplest possible Java webapp. “Real” applications are usually closer to the suggested 1.5x JVM heap guideline.


https://developers.redhat.com/blog/2017/04/04/openjdk-and-containers/:

Why is it when I specify -Xmx=1g my JVM uses up more memory than 1gb of memory?

Specifying -Xmx=1g is telling the JVM to allocate a 1gb heap. It’s not telling the JVM to limit its entire memory usage to 1gb. There are card tables, code caches, and all sorts of other off heap data structures. The parameter you use to specify total memory usage is -XX:MaxRAM. Be aware that with -XX:MaxRam=500m your heap will be approximately 250mb.

Java sees host memory size and it is not aware of any container memory limitations. It doesn't create memory pressure, so GC also doesn't need to release used memory. I hope XX:MaxRAM will help you to reduce memory footprint. Eventually, you can tweak GC configuration (-XX:MinHeapFreeRatio,-XX:MaxHeapFreeRatio, ...)


There is many types of memory metrics. Docker seems to be reporting RSS memory size, that can be different than "committed" memory reported by jcmd (older versions of Docker report RSS+cache as memory usage). Good discussion and links: Difference between Resident Set Size (RSS) and Java total committed memory (NMT) for a JVM running in Docker container

(RSS) memory can be eaten also by some other utilities in the container - shell, process manager, ... We don't know what else is running in the container and how do you start processes in container.

Java Memory Management for Java Virtual Machine (JVM), Why does my java process consume more memory than XMX? $ docker run -it--memory = 512m zsiegel:java-and-docker Number of processors: 4 Max memory: 119537664 bytes Now that we have adjusted the amount of memory available to the container itself the heap size is adjusted by the runtime.


TL;DR

The detail usage of the memory is provided by Native Memory Tracking (NMT) details (mainly code metadata and garbage collector). In addition to that, the Java compiler and optimizer C1/C2 consume the memory not reported in the summary.

The memory footprint can be reduced using JVM flags (but there is impacts).

The Docker container sizing must be done through testing with the expected load the application.


Detail for each components

The shared class space can be disabled inside a container since the classes won't be shared by another JVM process. The following flag can be used. It will remove the shared class space (17MB).

-Xshare:off

The garbage collector serial has a minimal memory footprint at the cost of longer pause time during garbage collect processing (see Aleksey Shipilëv comparison between GC in one picture). It can be enabled with the following flag. It can save up to the GC space used (48MB).

-XX:+UseSerialGC

The C2 compiler can be disabled with the following flag to reduce profiling data used to decide whether to optimize or not a method.

-XX:+TieredCompilation -XX:TieredStopAtLevel=1

The code space is reduced by 20MB. Moreover, the memory outside JVM is reduced by 80MB (difference between NMT space and RSS space). The optimizing compiler C2 needs 100MB.

The C1 and C2 compilers can be disabled with the following flag.

-Xint

The memory outside the JVM is now lower than the total committed space. The code space is reduced by 43MB. Beware, this has a major impact on the performance of the application. Disabling C1 and C2 compiler reduces the memory used by 170 MB.

Using Graal VM compiler (replacement of C2) leads to a bit smaller memory footprint. It increases of 20MB the code memory space and decreases of 60MB from outside JVM memory.

The article Java Memory Management for JVM provides some relevant information the different memory spaces. Oracle provides some details in Native Memory Tracking documentation. More details about compilation level in advanced compilation policy and in disable C2 reduce code cache size by a factor 5. Some details on Why does a JVM report more committed memory than the Linux process resident set size? when both compilers are disabled.

Why does my Java process consume more memory than Xmx , of VM Use Xmx and Xms flags while launching your VM and deliberately set their values to lower values than what might be needed and be on the lookout of OutOfMemoryError . If OutOfMemoryError happens which means you are over shooting the max VM size. The Heap Size was the same as our previous (local) launch: There is a very good analysis of Java memory usage in Docker containers, where you can find legible explanations and details on how


How to size correctly the Docker memory limit? Check the application by monitoring it for some-time. To restrict container's memory try using -m, --memory bytes option for docker run command - or something equivalant if you are running it otherwise like

docker run -d --name my-container --memory 500m <iamge-name>

can't answer other questions.

Reducing memory consumption in Java, In this post, I'll show you how to set memory limits for a basic Java web The JVM's footprint in RAM is significantly more than the heap size,  I have a project I'm writing (in Java) for a class where the prof says we're not allowed to use more than 200m I limit the stack memory to 50m (just to be absolutely sure) with -Xmx50m but according to top, it's still using 300m. I tried running Eclipse Memory Analyzer and it reports only 26m


Java needs a lot a memory. JVM itself needs a lot of memory to run. The heap is the memory which is available inside the virtual machine, available to your application. Because JVM is a big bundle packed with all goodies possible it takes a lot of memory just to load.

Starting with java 9 you have something called project Jigsaw, which might reduce the memory used when you start a java app(along with start time). Project jigsaw and a new module system were not necessarily created to reduce the necessary memory, but if it's important you can give a try.

You can take a look at this example: https://steveperkins.com/using-java-9-modularization-to-ship-zero-dependency-native-apps/. By using the module system it resulted in CLI application of 21MB(with JRE embeded). JRE takes more than 200mb. That should translate to less allocated memory when the application is up(a lot of unused JRE classes will no longer be loaded).

Here is another nice tutorial: https://www.baeldung.com/project-jigsaw-java-modularity

If you don't want to spend time with this you can simply get allocate more memory. Sometimes it's the best.

Docker memory resource limits and a heap of Java, Many developers are (or should be) aware that Java processes running default values for the garbage collector, heap size, and runtime compiler. I've executed a Kubernetes Pod with a memory limit of 512MB (using the more than 80% (​219.8 MiB) of the max allowed JVM memory size (241.7 MiB)”. where OffHeap consists of thread stacks, direct buffers, mapped files (libraries and jars) and JVM code itself; According to jvisualvm, committed Heap size is 136M (while just only 67M are "used"): MetaSapce size is 68M (67M are used): In other words, we had to explain 367M - (136M + 67M) = 164M of OffHeap memory.


Java inside docker: What you must know to not FAIL, Although the JVM can read cgroup memory limits and adapt memory usage It's important to note that the JVM uses more memory than just heap. Now the JVM is back in charge calculating the heap size, we just fine tune the parameters. 1073741824$ docker run --rm -m 1g openjdk:8-jdk sh -c "java  So again, you are going to lose memory. Off-heap allocations. If you happen to use off-heap memory, for example while using direct or mapped ByteBuffers yourself or via some clever 3rd party API then voila – you are extending your heap to something you actually cannot control via JVM configuration. JNI code.


JVM Memory Settings in a Container Environment - adorsys, Docker can set memory and CPU limitations that Java can't automatically detect Docker love for Java is in its way in newer versions of both OpenJ9 and OpenJDK 10 The JVM will claim more and more memory and go over this limit. this inside your container by setting -Xmx flags to limit the heap size. We have never seen problems in our heap memory usage, so we are reasonably sure that there are no obvious memory leaks. We’ve also verified this using VisualVM locally. As a result, we did not need to tweak the JVM flags that influence heap memory size (-Xms and -Xmx, the favourite Java developer flags in the universe).


Java and Docker, the limitations, We can overcome this issue by setting the correct heap size for our Java application. being killed when JVM tries to use more memory beyond the docker container memory. heap size (128MB) is lesser than the container memory(250MB). In this case, if your docker memory limit is 1GB, then JVM sets  For example if you limit your Docker container to get only 100MB of memory, this isn’t something ‘old’ Java was aware of. Java doesn’t see this limit. The JVM will claim more and more memory and go over this limit. Docker will then take action into its own hands and kill the process inside the container if too much memory is used!