shmseg kernel parameter

[Vito Sarducci]

A brief overview of what this kernel parameter does, default setting in a oracle server 8.1.6 running hp-ux 11.0 in 32 bit environment, and what is affected if changed?

Our vendor wants a lower number of this kernel parameter then what is already defaulted with the system install.

they want it set to 12 which I find very low? Some of our Oracle servers are set to 1024.

Please advise?

Vito

[A. Clay Stephenson]

Hi:

Rule Number 1) Ignore DBA'w when tuning kernels. Do they listen to you when tuning their database?

Depending upon what else is running on your box, lowering SHMSEG to 12 may kill some processes. This is the number of separate shared memory attachments allowed by a single process. 1024 is probably high but 12 is probably low. Somewhere in the range of 60-120 should keep everybody happy.

[Helen French · Accepted solution]

Hi,

Setting shmseg to 20 is not a good idea. One of my system running 32 bit 11.0 and have Oracle running on that, the shmseg is set to -120.

Normally when tuning kernel, never try to attempt reducing the Kernel Paremeter to a lower value than the default, unless until you are very sure !

For more about shmseg, check this out:

http://www.docs.hp.com/cgi-bin/otsearch/getfile?id=/hpux/onlinedocs/os/KCparam.ShMseg.html&searchterms=shmseg&queryid=20020228-125804

HTH,
Shiju

[Don Morris_1]

Just as another data point - there's no reason to lower shmseg unless you have misbehaving applications hogging shared memory (that's really the whole point of the tunable). shmseg doesn't cause any data structure or memory allocation based on it's value within the kernel - so it's completely harmless from a performance standpoint.

In other words - make them give you a good reason. :)

Don

[harry d brown jr]

Vito,

How many times have you been asked: "are you the priest on saturday night live?"

live free or die
harry

[Dennis J Robinson]

HP-GURU speaks:

All shared memory parameters:

shmmax - Max size of a single shared memory segment.
shmmni - Number of shared memory segments systemwide.
shmseg - Number of shared memory segments a single process may attach to.

There are some DBA's which I call "Physical DBA's" which will be able to tune the HP-UX kernel and also lay out a database and optimize the init.ora much better than a "logical dba".

init.ora settings are a black magic for most Oracle DBA's...