Cannot allocate shared memory and kernel.shmmax, kernel.shmall

Ok, this one is short but cool. After server update and reboot I noticed the zabbix-proxy (and later on found out that zabbix-agent also) didn’t start up. Running service zabbix-proxy start gives you OK, but status check later tells that non-OK and service is stopped.

Quick look into zabbix-proxy log file shows the following:

cannot allocate shared memory of size 16777216: [22] Invalid argument

Hmmm… Checking system memory and looking around I didn’t notice any problems or lack of resources, so a big of googling pointed me to check kernel’s shared memory configuration

]# sysctl -a | grep shmmax
kernel.shmmax = 0

And here 0 doesn’t mean unlimited, but literally zero! Ok, fine, but what’s in /etc/sysctl.conf?

 # Controls the maximum shared segment size, in bytes
kernel.shmmax=68719476736

Don’t ask me why the value is exactly what it is, it was there historically. Anyway, this is wrong, as it is bigger than 0. We need to change it!

]# echo 68719476736 > /proc/sys/kernel/shmmax 
]# sysctl -a |grep shmmax
kernel.shmmax = 0
]# sysctl -w kernel.shmmax=68719476736
kernel.shmmax = 68719476736
]# sysctl -a |grep shmmax
kernel.shmmax = 0

WTF? WTF? WTF? (retvals from my brain doing and seeing above). Again a bit of googling, and here we are:

  • After setting kernel parameter SHMMAX to a value larger than 4GB on a 32-bit Red Hat Enterprise Linux system, this value appears to be reset to 0.

Checking that the server has 2GB RAM, changed the value of shmmax to 2147483648 and repeating the above all worked out as expected with value being applied. Restarting my zabbix services, checking again, still no luck with a slightly different message this time:

cannot allocate shared memory of size 16777216: [28] No space left on device

Seriously?! Checking /etc/sysctl.conf one more time, I found that kernel.shmall has a bit value there as well, but 0 in real life. Adjusting it to match kernel.shmmax and restarting the services worked this time.

It’s a pity that RedHat knowledge base doesn’t make a hint about it, as the problem is common for SHMMAX and SHMALL.

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