SHMMAX or SHMALL error on Big Sur – Sketchpad – Dan Sketcher’s personal blog

When upgrading to Big Sur with Homebrew, you may find errors on starting Postgres where there’s memory errors.

2021-03-30 17:26:07.203 AEST [47951] FATAL: could not create shared memory segment: Cannot allocate memory
2021-03-30 17:26:07.203 AEST [47951] DETAIL: Failed system call was shmget(key=5432001, size=56, 03600).
2021-03-30 17:26:07.203 AEST [47951] HINT: This error usually means that PostgreSQL's request for a shared memory segment exceeded your kernel's SHMALL parameter. You might need to reconfigure the kernel with larger SHMALL.
The PostgreSQL documentation contains more information about shared memory configuration.
2021-03-30 17:26:07.204 AEST [47951] LOG: database system is shut down
stopped waiting
pg_ctl: could not start server

The short term (until reboot) solution lies here:

https://stackoverflow.com/questions/2017004/setting-shmmax-etc-values-on-mac-os-x-10-6-for-postgresql/10629164#10629164

Basically:

sudo sysctl -w kern.sysv.shmmax=12582912
sudo sysctl -w kern.sysv.shmall=12582912

But for a more permanent switch, since Catalina, editing /etc/sysctl.conf doesn’t work.

After further digging, the solution to this problem is to place a plist file into /Library/LaunchDaemons/ which runs and sets the shared memory parameters on boot. On Catalina this has to be preceded by disabling SIP, and remounting the / file system as writable, before installing the plist file.
https://developer.apple.com/forums/thread/669625

so, go to /Library/LaunchDaemons/, create a file (I called mine memory.plist, and make sure the owner is correct!), add the content below, and then run
sudo launchctl load memory.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
 <key>Label</key>
 <string>shmemsetup</string>
 <key>UserName</key>
 <string>root</string>
 <key>GroupName</key>
 <string>wheel</string>
 <key>ProgramArguments</key>
 <array>
  <string>/usr/sbin/sysctl</string>
  <string>-w</string>
  <string>kern.sysv.shmmax=268435456</string>
  <string>kern.sysv.shmmni=128</string>
  <string>kern.sysv.shmseg=32</string>
  <string>kern.sysv.shmall=65536</string>
 </array>
 <key>KeepAlive</key>
 <false/>
 <key>RunAtLoad</key>
 <true/>
</dict>
</plist>

Once you’ve rebooted you can confirm by running

$ sysctl -A kern.sysv.shmall
kern.sysv.shmall: 65536

$ sysctl -A kern.sysv.shmmax
kern.sysv.shmmax: 268435456