First, there are a lot of articles about JVM and container-awareness:

• https://medium.com/@jnsrikanth/docker-support-in-java-10-fbff28a31827
• https://blogs.oracle.com/java-platform-group/java-se-support-for-docker-cpu-and-memory-limits

In this post, I use Java 11, which means the default for garbage collector is supposed to be G1GC! Let's look at the defaults, which are automatically chosen by the JVM depending on memory size and provided CPUs.

Default GC Based on Docker CPU

$ docker run --cpus="2" openjdk:11-jre java -XX:+PrintFlagsFinal -version | grep -E "(MAX|UseSerialGC|UseG1GC|MaxHeapSize)"
   size_t MaxHeapSize   = 5200936960    {product} {ergonomic}
     bool UseG1GC       = true          {product} {ergonomic}
     bool UseSerialGC   = false         {product} {default}

$ docker run --cpus="1" openjdk:11-jre java -XX:+PrintFlagsFinal -version | grep -E "(MAX|UseSerialGC|UseG1GC|MaxHeapSize)"
   size_t MaxHeapSize    = 5200936960     {product} {ergonomic}
     bool UseG1GC        = false          {product} {default}
     bool UseSerialGC    = true           {product} {ergonomic}

Be aware of the number of CPUs in your Docker container; the type of garbage collector can be absolutely different.

Default GC Based on Docker Memory

$ docker run -m 2g openjdk:11-jre java -XX:+PrintFlagsFinal -version | grep -E "(MAX|UseSerialGC|UseG1GC|MaxHeapSize)"
   size_t MaxHeapSize     = 536870912      {product} {ergonomic}
     bool UseG1GC         = true           {product} {ergonomic}
     bool UseSerialGC     = false          {product} {default}

$ docker run -m 1g openjdk:11-jre java -XX:+PrintFlagsFinal -version | grep -E "(MAX|UseSerialGC|UseG1GC|MaxHeapSize)"
   size_t MaxHeapSize    = 268435456     {product} {ergonomic}
     bool UseG1GC        = false         {product} {default}
     bool UseSerialGC    = true          {product} {ergonomic}

In the snippets above, the JVM automatically changed the default garbage collector, depending on the provided memory size.

Default Heap Size Is Always 1/4 of Available Memory! Really?

$ docker run -m 512m openjdk:11-jre java -XX:+PrintFlagsFinal -version | grep -E "(MAX|UseSerialGC|UseG1GC|MaxHeapSize)"
   size_t MaxHeapSize     = 134217728     {product} {ergonomic}
     bool UseG1GC         = false         {product} {default}
     bool UseSerialGC     = true          {product} {ergonomic}

$ docker run -m 256m openjdk:11-jre java -XX:+PrintFlagsFinal -version | grep -E "(MAX|UseSerialGC|UseG1GC|MaxHeapSize)"
   size_t MaxHeapSize     = 132120576      {product} {ergonomic}
     bool UseG1GC         = false          {product} {default}
     bool UseSerialGC     = true           {product} {ergonomic}

From a certain amount of memory, the JVM stops choosing 1/4 of the heap size and starts making the ratio smaller and smaller to provide your application bigger heap memory.

Why SerialGC?

$ docker run -m 1g openjdk:11-jre java -XX:+PrintFlagsFinal -version | grep -E "(MAX|UseSerialGC|UseG1GC|MaxHeapSize)"
   size_t MaxHeapSize      = 268435456      {product} {ergonomic}
     bool UseG1GC          = false          {product} {default}
     bool UseSerialGC      = true           {product} {ergonomic}

$ docker run -m 1g openjdk:11-jre java -Xmx700m -XX:+PrintFlagsFinal -version | grep -E "(MAX|UseSerialGC|UseG1GC|MaxHeapSize)"
   size_t MaxHeapSize      = 734003200         {product} {command line}
     bool UseG1GC          = false             {product} {default}
     bool UseSerialGC      = true              {product} {ergonomic}

This is very weird, according to the example above, we should still have G1GC as a default, but know we can notice that JVM picked up SerialGC. One takeaway, you always have to try out what defaults were picked up by JVM and think if it's reasonable and suitable for you. You can notice that parameters defined by JVM are marked as ergonomic JVM options.

Thank you for reading my article and please take a look at the comments below. If you like being notified about new posts, then start following me on Twitter: @p_bouda.