use openj9 as the Java runtime to reduce memory usage

Dockerfile

@@ -1,4 +1,4 @@
-FROM eclipse-temurin:21.0.3_9-jre
+FROM ibm-semeru-runtimes:open-21-jre
 
 EXPOSE 8082
 
@@ -8,5 +8,5 @@ VOLUME /commafeed/data
 COPY commafeed-server/config.yml.example config.yml
 COPY commafeed-server/target/commafeed.jar .
 
-ENV JAVA_TOOL_OPTIONS -Djava.net.preferIPv4Stack=true -Xms20m -XX:+UseG1GC -XX:+UseStringDeduplication -XX:-ShrinkHeapInSteps -XX:G1PeriodicGCInterval=10000 -XX:-G1PeriodicGCInvokesConcurrent -XX:MinHeapFreeRatio=5 -XX:MaxHeapFreeRatio=10
+ENV JAVA_TOOL_OPTIONS -Djava.net.preferIPv4Stack=true -Xtune:virtualized -Xminf0.05 -Xmaxf0.1
 CMD ["java", "-jar", "commafeed.jar", "server", "config.yml"]

README.md

@@ -58,7 +58,7 @@ user is `admin` and the default password is `admin`.
 
 The Java Virtual Machine (JVM) is rather greedy by default and will not release unused memory to the
 operating system. This is because acquiring memory from the operating system is a relatively expensive operation.
-However, this can be problematic on systems with limited memory.
+This can be problematic on systems with limited memory.
 
 #### Hard limit
 
@@ -77,6 +77,15 @@ and [here](https://docs.oracle.com/en/java/javase/17/gctuning/factors-affecting-
 more
 information.
 
+#### OpenJ9
+
+The [OpenJ9](https://eclipse.dev/openj9/) JVM is a more memory-efficient alternative to the HotSpot JVM, at the cost of
+slightly slower throughput.
+
+IBM provides precompiled binaries for OpenJ9
+named [Semeru](https://developer.ibm.com/languages/java/semeru-runtimes/downloads/).
+This is the JVM used in the [Docker image](https://github.com/Athou/commafeed/blob/master/Dockerfile).
+
 ## Translation
 
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