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Old 11-16-2010, 01:55 PM   #1
pacavatar
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Cool How increasing RAM affects kernel parameters


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?

Thanks in advance!
 
Old 11-16-2010, 03:04 PM   #2
Tinkster
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Hi, welcome to LQ!


Increasing shmmax for Oracle will be necessary; whether or not that will
impact on your jboss depends on whether or not the machine is going into
swap or not.

What does
Code:
free -m
look like after the change?


Cheers,
Tink
 
Old 11-16-2010, 03:33 PM   #3
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Smile

Code:
total       used       free     shared    buffers     cached
Mem:         32133      31520        612          0        160      20343
-/+ buffers/cache:      11016      21117
Swap:         8191         99       8092
Code:
ipcs -lm

------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 8388608
max total shared memory (kbytes) = 13118188
min seg size (bytes) = 1
does this represent 50% of the previous 16gb, and now needs to be doubled to be 50% of 32gb?

Last edited by Tinkster; 11-16-2010 at 04:12 PM.
 
Old 11-16-2010, 04:26 PM   #4
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Quote:
Originally Posted by pacavatar View Post
Code:
total       used       free     shared    buffers     cached
Mem:         32133      31520        612          0        160      20343
-/+ buffers/cache:      11016      21117
Swap:         8191         99       8092
So the machine has nothing much to do atm ... :}


Quote:
Originally Posted by pacavatar View Post
Code:
ipcs -lm

------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 8388608
max total shared memory (kbytes) = 13118188
min seg size (bytes) = 1
does this represent 50% of the previous 16gb, and now needs to be doubled to be 50% of 32gb?
That depends on how big the SGA needs to become. Blindly
doubling the value may not be a clever move.


Cheers,
Tink
 
Old 11-16-2010, 06:04 PM   #5
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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.

Thanks!
 
Old 11-16-2010, 07:13 PM   #6
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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
 
Old 11-16-2010, 07:21 PM   #7
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What does
Code:
grep  kernel.shmmax /etc/sysctl.conf
say?

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?


Cheers,
Tink
 
Old 11-17-2010, 07:09 PM   #8
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it's at 8gb...half of the previous 16 gb total. That is why I assumed that it is advisable to double it to 16gb esp as we would like to bump the SGA to 12 gb and leave some room for the PGA.

kernel.shmmax = 8589934592

thanks for input on this!
 
  


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