java环境内存不够导致swap
云服务器默认不支持创建swap，所以部分场景下需要 创建一个文件让文件成为swap。
#1.创建一个文件块
[root@oldboyedu01-nb ~]# dd if=/dev/zero of=/tmp/100m bs=1M count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.935927 s, 112 MB/s
[root@oldboyedu01-nb ~]# file /tmp/100m
/tmp/100m: data
#2.变成swap
[root@oldboyedu01-nb ~]# mkswap /tmp/100m
mkswap: /tmp/100m: warning: don't erase bootbits sectors
on whole disk. Use -f to force.
Setting up swapspace version 1, size = 102396 KiB
no label, UUID=1b2aea27-a092-4345-806f-b95b8d33873a
[root@oldboyedu01-nb ~]# file /tmp/100m
/tmp/100m: Linux/i386 swap file (new style) 1 (4K pages) size 25599 pages
#3.swap生效
[root@oldboyedu01-nb ~]# swapon /tmp/100m
[root@oldboyedu01-nb ~]# free -h
total used free shared buffers cached
Mem: 3.9G 398M 3.5G 248K 65M 134M
-/+ buffers/cache: 198M 3.7G
Swap: 867M 0B 867M
[root@oldboyedu01-nb ~]# swapon -s
Filename Type Size Used Priority
/dev/sda2 partition 786428 0 -1
/tmp/100m file 102396 0 -2
#4.swap永久生效
###方法1. /etc/rc.local
swapon /tmp/100m
###方法2. /etc/fstab
/tmp/100m swap swap defaults 0 0
[root@oldboyedu01-nb ~]# free -h
total used free shared buffers cached
Mem: 3.9G 398M 3.5G 248K 65M 134M
-/+ buffers/cache: 199M 3.7G
Swap: 867M 0B 867M
Linux的特点：会把使用过的命令和数据临时存放在内存中 cache和buffer，所以上面真实可用空间为3.7G。
