Semaphore scaling

Doug White dwhite at
Thu Mar 4 11:20:29 PST 2004

Don't crosspost lists. Thanks.

On Thu, 4 Mar 2004 mikej at wrote:

> I am running an application (Plesk) on a 4.9 system that is eating up
> all the semaphores and as a result some processes can’t run. I have
> contacted the software company, and they asked me to increase the
> semaphore limits in the kernel. However their suggested settings seemed
> very unproportional to me.
> So my question is, how should I adjust the values, while keeping things
> in proportion? I wish to increase the default semaphore limit of 10 to
> 20. Can I simply double the semaphore related values?

The semaphore tunables aren't quite as obvious as they look. There are
limits on the size of various clusters of things whos usage is heavily
application-dependent. That being said, FreeBSD traditionally has very low
SYSVSHM/SEM/MSG limits, so doubling them should be safe.  I suggest using
the ipcs(8) command to monitor the usage and adjust accordingly.

The Plesk-supplied limits seem OK to me too.  I've put Oracle-sized
values in there and haven't had any bad effects .. yet :)

Note that you can adjust these with loader tunables as well, if you don't
want to rebuild the kernel.  They have the same name as the sysctls that
show the current values in kern.ipc.

Doug White                    |  FreeBSD: The Power to Serve
dwhite at          |

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