Where does Linux set the default values for SHMMAX?

kernel memory shared-memory

7,118

On the fly you can just echo into /proc/sys/kernel/shmmax!

# echo 20446744073692774399 > /proc/sys/kernel/shmmax

But most people edit /etc/sysctl.conf with a line similar to:

kernel.shmmax=your_new_value_here

See the sysctl(8) man page.

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Brandon Condrey

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Updated on September 18, 2022

Comments

Brandon Condrey over 1 year
I'm just wondering where these values are being set and what they default to? Mine is currently 18446744073692774399. I didn't set it anywhere that I can see.
```
$ cat /proc/sys/kernel/shmmax 
18446744073692774399

$ sysctl kernel.shmmax
kernel.shmmax = 18446744073692774399
```
Brandon Condrey almost 7 years

That's not the question. Where are the defaults set? That is, if you don't write to shmmax yourself, what does it default to?
Deathgrip almost 7 years

On some Linux distros (RHEL 5&6 I believe is one) it is set in /etc/sysctl.conf. But I think the default is set in the kernel code to the theoretical limit (guessing that's 2^64 - 1?).