Linux Reports -bash: fork: Cannot Allocate Memory

There was a test machine this morning. When I checked with ps, I found that the command could not be executed and it returned "-bash: fork: Unable to allocate memory". Do I have to restart the server? Suddenly I found that I connected a lot of terminals and then disconnected Open a terminal, and then you can type commands in the terminal
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[root@172.16.31.105 /home/www/test]# free -m
             total used free shared buffers cached
Mem: 15979 8025 7954 0 218 1109
-/+ buffers/cache: 6697 9282
Swap: 0 0 0

There is still so much memory. . . I was speechless, and then I started typing the command again, but the problem was reported just now, "-bash: fork: unable to allocate memory", and there was a problem with another terminal connection.

[root@172.16.31.102 ~]# ssh 172.16.31.105
shell request failed on channel 0

When I saw this, I googled it. The Internet said that the kernel pid parameter of the old version of the system is relatively small (the default setting is 32768), so I want to verify that there are more terminals connected before, so there is no need to restart the server, of course I also got lucky, and then I disconnected another terminal, here I ps a bit

[root@172.16.31.105 ~]# ps
  PID TTY TIME CMD
32775 pts/1 00:00:00 bash
32765 pts/1 00:00:00 ps

Obviously, the pid number of ps is obviously close to the default pid of the kernel. Modify the kernel pid immediately and let it take effect. The command is as follows:

echo "kernel.pid_max=64000" >>/etc/sysctl.conf

sysctl -p

After it takes effect, the terminal can directly connect to this machine immediately.

To verify if this is the problem (just treat it as dead)

Find another machine, connect a few more terminals first, and then change the kernel pid to a smaller value to approximate the current maximum pid. After the change takes effect, the problem will come out and report "-bash: fork: unable to allocate memory". Find a way to change the kernel parameters quickly

Note: Some machines use the pid more seriously. If you type the command multiple times, it will report "-bash: fork: unable to allocate memory". Therefore, you can only restart the server. If you are lucky, you can see it by typing a few more commands.