Configuration_System.md

# The system configuration

This section focuses on the configuration of the system (JVM/OS)

## **1 JVM Options** ##

The latest released version of JDK 1.8 is recommended. Set the same Xms and Xmx value to prevent the JVM from resizing the heap for better performance. A simple JVM configuration is as follows:

    -server -Xms8g -Xmx8g -Xmn4g

Direct ByteBuffer memory size setting. Full GC will be triggered when the Direct ByteBuffer up to the specified size:

    -XX:MaxDirectMemorySize=15g

If you don’t care about the boot time of RocketMQ broker, pre-touch the Java heap to make sure that every page will be allocated during JVM initialization is a better choice. Those who don’t care about the boot time can enable it:
    
    -XX:+AlwaysPreTouch

Disable biased locking maybe reduce JVM pauses:

    -XX:-UseBiasedLocking

As for garbage collection, G1 collector with JDK 1.8 is recommended:

    -XX:+UseG1GC -XX:G1HeapRegionSize=16m 
    -XX:G1ReservePercent=25
    -XX:InitiatingHeapOccupancyPercent=30

These GC options looks a little aggressive, but it’s proved to have good performance in our production environment

Don’t set a too small value for -XX:MaxGCPauseMillis, otherwise JVM will use a small young generation to achieve this goal which will cause very frequent minor GC.So use rolling GC log file is recommended:
    
    -XX:+UseGCLogFileRotation 
    -XX:NumberOfGCLogFiles=5 
    -XX:GCLogFileSize=30m
    
If write GC file will increase latency of broker, consider redirect GC log file to a memory file system:
    
    -Xloggc:/dev/shm/mq_gc_%p.log123

## 2 Linux Kernel Parameters ##

There is a os.sh script that lists a lot of kernel parameters in folder bin which can be used for production use with minor changes. Below parameters need attention, and more details please refer to documentation for /proc/sys/vm/*.




- **vm.extra_free_kbytes**, tells the VM to keep extra free memory between the threshold where background reclaim (kswapd) kicks in, and the threshold where direct reclaim (by allocating processes) kicks in. RocketMQ uses this parameter to avoid high latency in memory allocation. (It is specific to the kernel version）



- **vm.min_free_kbytes**, if you set this to lower than 1024KB, your system will become subtly broken, and prone to deadlock under high loads.





- **vm.max_map_count**, limits the maximum number of memory map areas a process may have. RocketMQ will use mmap to load CommitLog and ConsumeQueue, so set a bigger value for this parameter is recommended.



- **vm.swappiness**, define how aggressive the kernel will swap memory pages. Higher values will increase agressiveness, lower values decrease the amount of swap. 10 is recommended for this value to avoid swap latency.



- **File descriptor limits**, RocketMQ needs open file descriptors for files(CommitLog and ConsumeQueue) and network connections. We recommend setting  655350 for file descriptors.



- **Disk scheduler**, the deadline I/O scheduler is recommended for RocketMQ, which attempts to provide a guaranteed latency for requests.