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We just doubled the RAM on our RHEL 4 production servers...Oracle from 16gb to 32gb and Apache/JBoss from 8 to 32 gb. I am trying to figure out how to get the biggest bang from the increased RAM. Should kernel.shmmax be upped to be 50% of the 32gb....from its current 8gb? I need to create the largest possible SGA and was unable to do so, apparently because the kernel.shmmax value had gone unchanged.
And for the JBoss web servers, also RHEL 4, and particular kernel changes which can take advantage of the increased RAM and provide an immediate performance improvement?
so if the SGA is to be 12 gb, how big should the shmmax become. All this server does is support Oracle 10g (RAC). At times it is very busy (evenings) and at other times ...not so much My objective is to fully utilize the new memory and provide demonstrably faster backend processing.
You questions are highly system dependent. SGA should not be greater, or even near in my opinion, 75% of real memory. You must be using the hugemem kernel currently, because you can see 32GB of ram. Using the deadline I/O scheduler, rather than CFQ, in the kernel is better for raw I/O throughput. CFQ is a great compromise solution, but not for database use. I use this page for reference: http://www.puschitz.com/TuningLinuxForOracle.shtml
At the end of the day I'm not really a DBA (Linux bod' here), but Linux
can normally make good use of RAM for caching, which happens to be a good
thing for pretty much any application. Maybe we have some Oracle DBAs
reading here who can give you a more targeted answer?