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#Disable heuristic rules
-XX:+UseCMSInitiatingOccupancyOnly

#Reduce Old Gen threshold
-XX:CMSInitiatingOccupancyFraction=70

# Use System.gc() concurrently in CMS
-XX:+ExplicitGCInvokesConcurrent

# Disable System.gc() for RMI, for 10000 hours
-Dsun.rmi.dgc.client.gcInterval=36000000000
-Dsun.rmi.dgc.server.gcInterval=36000000000

The Z Garbage Collector

The Z Garbage Collector (ZGC) is a scalable low latency garbage collector for Java 12. ZGC performs all expensive work concurrently, without stopping the execution of application threads for more than 10ms, which makes is suitable for applications which require low latency and/or use a very large heap. You should pay attention to ZGC requires more processor resources compared to CMS GC.

Example installation of ZGC with OpenJDK version 12:

1. Download the latest release of OpenJDK 12 from URL http://jdk.java.net/12/:

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wget https://download.java.net/java/GA/jdk12.0.2/e482c34c86bd4bf8b56c0b35558996b9/10/GPL/openjdk-12.0.2_linux-x64_bin.tar.gz

2. Extract the OpenJDK to the folder and move its contents to the working directory:

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tar xvf openjdk-12.0.2_linux-x64_bin.tar.gz
mv jdk-12.0.2 /usr/java/jdk-12.0.2

3. Create symbolic links to OpenJDK 12:

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ln -sf /usr/java/jdk-12.0.2 /usr/java/default
ln -sf /usr/java/default/bin/java /usr/bin/java
ln -sf /usr/java/default/bin/jstack /usr/bin/jstack
ln -sf /usr/java/default/bin/jcmd /usr/bin/jcmd
ln -sf /usr/java/default/bin/jmap /usr/bin/jmap

4. Verify your Java installation:

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java --version

openjdk 12.0.2 2019-07-16
OpenJDK Runtime Environment (build 12.0.2+10)
OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode, sharing)

5. Install the WCS (if required).

6. Tune the wcs-core.properties (for example, allocating 24G under memory heap):

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# ZGC
-XX:+UnlockExperimentalVMOptions -XX:+UseZGC -Xms24g -Xmx24g -XX:+UseLargePages -XX:ZPath=/hugepages

7. Сonfigure ZGC according to the recommendations (the number of memory pages (2048K each) with a margin of a quarter more than that obtained from calculating the memory for heap (1.25 * 24G / 2M)) and add the required parameters in the server startup:

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mkdir /hugepages
echo "echo 13051 >/sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages" >>/etc/rc.local
echo "mount -t hugetlbfs -o uid=0 nodev /hugepages" >>/etc/rc.local
chmod +x /etc/rc.d/rc.local
systemctl enable rc-local.service
systemctl restart rc-local.service

8.  After restarting WCS, the gc-core.log log files show the periodic operation of the garbage collector. To understand the working model of Z Garbage Collector, you can read the presentation.

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Heap memory tuning

Many objects with data are created and destroyed in memory while streaming. Therefore it is recommended to allocate at least 1/2 of server physical memory for Java memory heap. For example, if server RAM is 32 Gb, then it is recommended to allocate 16 Gb with the following settings in wcs-core.properties file

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