按照官网教程https://rocketmq.apache.org/docs/quick-start/，输入命令，结果提示没有此目录，namesrv应该是启动失败了，下面开始排查过程。
> unzip rocketmq-all-4.8.0-source-release.zip > cd rocketmq-all-4.8.0/ > mvn -Prelease-all -DskipTests clean install -U > cd distribution/target/rocketmq-4.8.0/rocketmq-4.8.0 > nohup sh bin/mqnamesrv & > tail -f ~/logs/rocketmqlogs/namesrv.log
1.使用tail -f nohup.out命令，发现提示这个报错信息，这更加证实我的想法，果然启动报错了，错误日志保存在了hs_err_pid15988.log文件中。
# An error report file with more information is saved as:
# /usr/soft/rocketmq-all-4.8.0-source-release/distribution/target/rocketmq-4.8.0/rocketmq-4.8.0/hs_err_pid15988.log
Java HotSpot(TM) 64-Bit Server VM warning: Using the DefNew young collector with the CMS collector is deprecated and will likely be removed in a future release
Java HotSpot(TM) 64-Bit Server VM warning: UseCMSCompactAtFullCollection is deprecated and will likely be removed in a future release.
Java HotSpot(TM) 64-Bit Server VM warning: INFO: os::commit_memory(0x00000006ec800000, 2147483648, 0) failed; error='Cannot allocate memory' (errno=12)
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (mmap) failed to map 2147483648 bytes for committing reserved memory.
# An error report file with more information is saved as:
# /usr/soft/rocketmq-all-4.8.0-source-release/distribution/target/rocketmq-4.8.0/rocketmq-4.8.0/hs_err_pid16792.log
2.使用vi hs_err_pid15988.log命令查看文件，发现内存不足，
#
#There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (mmap) failed to map 2147483648 bytes for committing reserved memory.
# Possible reasons:
# The system is out of physical RAM or swap space
# The process is running with CompressedOops enabled, and the Java Heap may be blocking the growth of the native heap
# Possible solutions:
# Reduce memory load on the system
# Increase physical memory or swap space
# Check if swap backing store is full
# Decrease Java heap size (-Xmx/-Xms)
# Decrease number of Java threads
# Decrease Java thread stack sizes (-Xss)
# Set larger code cache with -XX:ReservedCodeCacheSize=
# This output file may be truncated or incomplete.
#
# Out of Memory Error (os_linux.cpp:2756), pid=16792, tid=0x00007f51850e3700
3.查看内存，free-h命令，发现available内存只有1.0G了，加上rocketmq启动要4个g，所以就内存不足了。
total used free shared buff/cache available
Mem: 1.8G 654M 411M 1.6M 771M 1.0G
Swap: 0B 0B 0B
4.解决办法就是改配置
vi runserver.sh，修改第一行，内存改小点。
#===========================================================================================
# JVM Configuration
#===========================================================================================
#JAVA_OPT="${JAVA_OPT} -server -Xms4g -Xmx4g -Xmn2g -XX:MetaspaceSize=128m -XX:MaxMetaspaceSize=320m"
JAVA_OPT="${JAVA_OPT} -server -Xms256m -Xmx256m -Xmn125m -XX:MetaspaceSize=128m -XX:MaxMetaspaceSize=320m"
JAVA_OPT="${JAVA_OPT} -XX:+UseConcMarkSweepGC -XX:+UseCMSCompactAtFullCollection -XX:CMSInitiatingOccupancyFraction=70 -XX:+CMSParallelRemarkEnabled -XX:SoftRefLRUPolicyMSPerMB=0 -XX:+CMSClassUnloadingEnabled -XX:SurvivorRatio=8 -XX:-UseParNewGC"
JAVA_OPT="${JAVA_OPT} -verbose:gc -Xloggc:/dev/shm/rmq_srv_gc.log -XX:+PrintGCDetails"
JAVA_OPT="${JAVA_OPT} -XX:-OmitStackTraceInFastThrow"
JAVA_OPT="${JAVA_OPT} -XX:-UseLargePages"
JAVA_OPT="${JAVA_OPT} -Djava.ext.dirs=${JAVA_HOME}/jre/lib/ext:${BASE_DIR}/lib"
#JAVA_OPT="${JAVA_OPT} -Xdebug -Xrunjdwp:transport=dt_socket,address=9555,server=y,suspend=n"
JAVA_OPT="${JAVA_OPT} ${JAVA_OPT_EXT}"
JAVA_OPT="${JAVA_OPT} -cp ${CLASSPATH}"
vi runbroker.sh,也是修改第一行，内存改小点。
#===========================================================================================
# JVM Configuration
#===========================================================================================
#JAVA_OPT="${JAVA_OPT} -server -Xms8g -Xmx8g -Xmn4g"
JAVA_OPT="${JAVA_OPT} -server -Xms256m -Xmx256m -Xmn125m"
JAVA_OPT="${JAVA_OPT} -XX:+UseG1GC -XX:G1HeapRegionSize=16m -XX:G1ReservePercent=25 -XX:InitiatingHeapOccupancyPercent=30 -XX:SoftRefLRUPolicyMSPerMB=0 -XX:SurvivorRatio=8"
JAVA_OPT="${JAVA_OPT} -verbose:gc -Xloggc:/dev/shm/mq_gc_%p.log -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCApplicationStoppedTime -XX:+PrintAdaptiveSizePolicy"
JAVA_OPT="${JAVA_OPT} -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=5 -XX:GCLogFileSize=30m"
JAVA_OPT="${JAVA_OPT} -XX:-OmitStackTraceInFastThrow"
JAVA_OPT="${JAVA_OPT} -XX:+AlwaysPreTouch"
JAVA_OPT="${JAVA_OPT} -XX:MaxDirectMemorySize=15g"
JAVA_OPT="${JAVA_OPT} -XX:-UseLargePages -XX:-UseBiasedLocking"
JAVA_OPT="${JAVA_OPT} -Djava.ext.dirs=${JAVA_HOME}/jre/lib/ext:${BASE_DIR}/lib"
#JAVA_OPT="${JAVA_OPT} -Xdebug -Xrunjdwp:transport=dt_socket,address=9555,server=y,suspend=n"
JAVA_OPT="${JAVA_OPT} ${JAVA_OPT_EXT}"