今天服务器提示不可用，查了了redis日志发现：
cat /data/redis/redis-server.log
[1316] 17 Jun 13:49:28.032 * 1 changes in 900 seconds. Saving...
[1316] 17 Jun 13:49:28.032 # Can't save in background: fork: Cannot allocate memory
[1316] 17 Jun 13:49:34.059 * 1 changes in 900 seconds. Saving...
[1316] 17 Jun 13:49:34.059 # Can't save in background: fork: Cannot allocate memory
[1316] 17 Jun 13:49:40.069 * 1 changes in 900 seconds. Saving...
[1316] 17 Jun 13:49:40.069 # Can't save in background: fork: Cannot allocate memory
[1316] 17 Jun 13:49:46.098 * 1 changes in 900 seconds. Saving...
[1316] 17 Jun 13:49:46.098 # Can't save in background: fork: Cannot allocate memory
[1316] 17 Jun 13:49:52.026 * 1 changes in 900 seconds. Saving...
[1316] 17 Jun 13:49:52.026 # Can't save in background: fork: Cannot allocate memory
因为Redis默认fork出一个进程来进行持久化到磁盘的工作，以防止主进程假死（大数据量时）。但是当主进程占用内存很大时，fork进程就不一定能够成功，所以出现这个错误。
修改方法：
# vim /etc/sysctl.conf
vm.overcommit_memory = 1
sysctl -p
Linux内核会根据参数vm.overcommit_memory参数的设置决定是否放行。
overcommit_memory文件指定了内核针对内存分配的策略，其值可以是0、1、2。
0， 表示内核将检查是否有足够的可用内存供应用进程使用；如果有足够的可用内存，内存申请允许；否则，内存申请失败，并把错误返回给应用进程。
1， 表示内核允许分配所有的物理内存，而不管当前的内存状态如何。
2， 表示内核允许分配超过所有物理内存和交换空间总和的内存
https://stackoverflow.com/questions/11752544/redis-bgsave-failed-because-fork-cannot-allocate-memory
