Summary - Shared Memory

From: McIntire, John (
Date: Wed Oct 18 2000 - 10:42:04 CDT

Thanks to the following for their great responses as it helped

Kevin Buterbaugh []

Shared memory (shminfo_shmmax) is intended to be physical memory locked for
the use of (typically) a database. It does not include swap space, but
setting this kernel parameter too high can cause a box to make heavy use of
it's swap space for the other processes it needs to run. We generally leave
anywhere from 128 - 256 MB of the physical memory in a box for the kernel
and other processes (i.e. on a system with 512 MB RAM, we never set
shminfo_shmmax to more than 400 MB, on a bigger box, we leave even more than

If this value is set to high, then all the other processes on the box have
to compete for the remaining physical memory, which generally does lead to
paging to the swap device(s). On a pure database server I'd set the
parameter to whatever you think is appropriate, then monitor the scan rate
with sar or vmstat. Under Solaris 2.7 and earlier, if the scan rate exceeds
273 pages / second, then you've got the parameter set too high. Under
Solaris 2.8, if the scan rate is much above zero, again, you've got it set
too high. Hope this helps...

Eugene Kramer [ekramer@Remedy.COM]

Jarrett Carver []

Shared Memory is not only memory used for a database. Shared memory is a
chunk of memory that can be accessed by other processes to prevent the
redundant loading of the same thing, therefore using more memory than is
needed. Also used with shared libraries, etc..

Check out the following article for more info on shared memory.

        John McIntire


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