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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 file


Garbage collector tuning

Garbage collector (GC) is an important part of Java VM. When GC is running, it dramatically increases the server load and may stop another tasks execution, therefore it is recommended to minimize GC launch with the following settings in file

#Disable heuristic rules

#Reduce Old Gen threshold

# Use System.gc() concurrently in CMS

# Disable System.gc() for RMI, for 10000 hours

Concurrent Mark Sweep (CMS) Garbage Collector

The Concurrent Mark Sweep (CMS) collector is designed for applications that prefer shorter garbage collection pauses and that can afford to share processor resources with the garbage collector while the application is running. This collector should be considered for any application with a low pause time requirement.

1. Сonfigure CMS GC in the (for example, allocating 24G under memory heap and tuning the NewSize and MaxNewSize parameters for control the new generation’s minimum and maximum size, regulate the new generation size by setting these parameters equal. In general, keep the eden size between one fourth and one third the maximum heap size)

# Used CMS GC
-XX:+UseConcMarkSweepGC -Xms24g -Xmx24g -XX:NewSize=6144m -XX:MaxNewSize=6144m

# Log

2. After restarting WCS in the gc-core.log log files we see the result of the garbage collector. The output may vary depending on the installed version of Java. For example:

openjdk version "1.8.0_222":

openjdk version "12.0.2":

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


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

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:

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

4. Verify your Java installation:

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 (for example, allocating 24G under memory heap):

-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:

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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