解决Nexus由于内存不足，造成启动失败问题（3.X版本）
1，问题描述
（1）之前在服务器上部署了个 Nexus 服务，突然无法访问（http://IP:8081/），重启服务依旧没能解决问题。执行如下命令结果为空，说明 8081 端口未被占用，也就是说 Nexus 服务实际上并没有启动。
lsof -i:8081
（2）在 Nexus 的 bin 文件夹下执行如下命令查看服务状态：
（3）显示如下信息，说明由于内存不足造成的：
（2）修改 Nexus 的内存配置即可：
./nexus status
（3）显示如下信息，说明由于内存不足造成的：
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (mmap) failed to map 419430400 bytes for committing reserved memory.
# Possible reasons:
# The system is out of physical RAM or swap space
# In 32 bit mode, the process size limit was hit
# Possible solutions:
# Reduce memory load on the system
# Increase physical memory or swap space
# Check if swap backing store is full
# Use 64 bit Java on a 64 bit OS
# 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:2640), pid=9927, tid=0x00007fdb4a672700
#
# JRE version: (8.0_151-b12) (build )
# Java VM: Java HotSpot(TM) 64-Bit Server VM (25.151-b12 mixed mode linux-amd64 compressed oops)
# Failed to write core dump. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (mmap) failed to map 419430400 bytes for committing reserved memory.
# Possible reasons:
# The system is out of physical RAM or swap space
# In 32 bit mode, the process size limit was hit
# Possible solutions:
# Reduce memory load on the system
# Increase physical memory or swap space
# Check if swap backing store is full
# Use 64 bit Java on a 64 bit OS
# 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:2640), pid=9927, tid=0x00007fdb4a672700
#
# JRE version: (8.0_151-b12) (build )
# Java VM: Java HotSpot(TM) 64-Bit Server VM (25.151-b12 mixed mode linux-amd64 compressed oops)
# Failed to write core dump. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
2，解决办法
（1）编辑 nexus.vmoptions 文件（在 Nexus 文件夹中的 bin 目录下）：
vi nexus.vmoptions
（2）修改 Nexus 的内存配置即可：
-Xms128M -Xmx128M -XX:MaxDirectMemorySize=256M
