upper limit of shmmax kernel parameter value

[Jdamian]

I checked the shmmax kernel parameter in my HPUX 11.00 system and found its value is 2048 MB.

I'm confused because my docs about kernel parameters tell its maximum value is 1792 MB.

Is that certain ?
Is it dangerous to keep it ?

[James R. Ferguson]

Hi:

An excellent guide to kernel parameters, their values and their affects, is this document:

http://docs.hp.com/hpux/onlinedocs/os/KCparams.OverviewAll.html

'shmmax' defines the system-wide maximum allowable shared memory segment size in bytes. Its value cannot exceed the maximum available swap space. The value you choose should be based on your needs.

...JRF...

[Darrel Louis]

Hi,

It's minimum value is 2048.

Check on the following page:
http://www.docs.hp.com//hpux/onlinedocs/os/KCparam.ShMmax.html

Good Luck

[Shannon Petry · Accepted solution]

I have not seen a minimum of 2048.....but then again.....It may depend on the release your running. Was that in Megabytes or Kbytes?

Anyway, here's a pretty descent rule of thumb, and easy to remember...
shmmax should be <= 80%(RAM+SWAP)
maxtsize should be <= 80%(RAM)
maxdsize and maxssize should be <= 80%(RAM+SWAP)

Most of the time, you will reach a kernel limit before you can hit these heights on new systems.... I.E. 1GB Swap and 2GB RAM is more than kernel max using the above formulas...but the 64bit features in 11.0 can go higher.

It is worth noting too, that it depends on what the systems are doing whether or not these params should be set at all. I.E. A web server needs open files lifted, and ninode lifted, but should not have to worry about memory allocation chunks so largs. A database server, or CAD/FEA server will need to be able to grab those huge chunks of memory, but may not have to open so many files.....



Hope it helps a bit
Shannon