FAQ
Hi Jon,

Happy New Year! I did a round of testing 10 - 12 years ago on Solaris, HP-UX and
Digital Unix (man I miss my Dec Alpha 8200 series...:( ) and did not find ANY
differences in OS or Oracle performance when configuring the OS with varying
values of SHMMAX(1GB, 4GB, 8GB and 16GB if memory serves me right). Too many
mind altering beverages in 10 years to keep those memory cells alive;)...digging deep from my archives;)

I do remember that the same workload was run on all settings and the workload
was traced. Trace output did not show any significant difference in any of the
elapsed times of the SQL. At the OS level, a sar -u output did not show any
significant increase in numbers under the %sys column for lower values of
SHMMAX. Nor did we observe any major increase in any virtual memory statistics
in the vmstat output. But way back then, the systems I played with had about
16GB (may be 32GB on Digital Unix) of memory.

Might be interesting to find out whether your 64GB Linux system today
demonstrates any significant increase in %sys, when you run the same workload
with a 1GB vs. say 32GB SHMMAX with a 32GB SGA. Assuming that technology (OS
memory management and Oracle Buffer Cache Management) has only positively
progressed in the last 10 years, I would probably speculate that smaller SHMMAX
values will not have a significant negative impact on performance. But a test is
worth a thousand speculations. A few key factors to keep in mind for your
tests:

Ensure that the SGA+PGA memory settings are reasonable and do not push the OS
in memory starvation mode.
Lock the SGA in memory to quieten the paging daemon to eliminate any
unnecessary paging.
Do not use Automatic SGA management just to keep the comparison clean for
your initial test runs. Turn this on in your later tests and see whether it is a
factor.
Enable direct I/O to ensure that the OS does not allocate a large filesystem
buffer cache and muddy the waters.

Keep us posted,

Cheers,

Gaja

Gaja Krishna Vaidyanatha,
Founder/Principal, DBPerfMan Inc.,
http://www.dbperfman.com
Phone - 001-(650)-743-6060
Co-author:Oracle Insights:Tales of the Oak Table -
http://www.apress.com/book/bookDisplay.html?bID=314
Co-author:Oracle Performance Tuning 101 -
http://www.amazon.com/gp/reader/0072131454/ref=sib_dp_pt/102-6130796-4625766

From: "CRISLER, JON A (ATTCORP)"
To: oracle-l@freelists.org
Sent: Thu, January 6, 2011 9:25:59 PM
Subject: Linx and kernel config shmmax

Is there any advantage or disadvantage to having really large or many smaller
shared memory segments ?


Lets say I have a 64gb machine, and I set up shmmax to support at least 64gb,
maybe even more. My db does not use anything close to 64g of memory. Is there
any drawbacks to this behavior?


What if I have a really large db, but I force it to use many smaller 4g or 8gb
segments to make up the entire sga ? We did some very limited internal testing
on this and could not come up with a firm opinion on which is better.

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