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Thread: Determining Shared Memory Parameters

  1. #1
    Join Date
    Feb 2001
    HI UNix/Linux DBA's
    As you know that Oracle Provides default values for Sahred Memory parameters to be set in SHMPARAM.H and SEM.H while installation Of Oracle.
    I have three questions reagrding them.
    1)How should one determine the optimal value of such parameters for his system?
    2)How these parameters affect the performace of the system if default values are considered while installation?
    3)Is there any method of changing the parametres after installation of the system?That IS,I feel that these are the values for these parameters which should have been set instead of the default values..and now I want to change these default values...Is there Any method..or the database server will need to be reinstalled?

  2. #2
    Join Date
    Apr 2000
    Baltimore, MD
    1. ask your Unix admin.
    2. I don't think this would affect the performance, ask this to your Unix admin anyways. I somehow suspect that increasing them MAY allocate unnecessary resources.
    3. You can change the parameters, however it is not advisable to do so unless and until required. For example, the file containing these values (in Solaris) is the /etc/system file. If you change these values, you have to bounce the server in order for them to take effect.

    [Edited by Halo on 04-19-2001 at 10:38 AM]

  3. #3
    Join Date
    Oct 2000
    Saskatoon, SK, Canada
    The shared memory parameters are some what critical to the database functionality on the Unix flavoured systems.

    1. The optimal size depends on the RAM you can support and
    the size of your instances' SGA, which majorly contibute
    to the allocation during runtime. So have an idea of
    what your instance size is going to be and the number of
    processes that it is going to support. This contributes
    to the allocation of Semaphores.

    2. It would be fine, if your instance is going to stick
    within the limit of these parameters and if it were to
    exceed you would come to know at instance start time.
    There are formulas that you could use from the oracle's
    installation manual to calculate the required SEMMNS and

    3. Once these system parameters have been put in place,
    you cannot change during runtime (to say in different way, that if
    you get to change those parameters after boot,
    you would require to reboot your system to get the
    new values take effect). Since these are of static in
    nature, it would require a bounce. on your server. You wouldn't require
    a reinstall if you were to change these parameters.

    Hope this would help you.

    [Edited by sambavan on 04-19-2001 at 11:06 AM]

    Life is a journey, not a destination!

  4. #4
    Join Date
    Feb 2001
    Thanx SambaVan,
    I appreciate your help earnestly.
    Actually I was concentrating on some performance Issues and Aftre trying My luck in a lot of things,I just thaught the real culprit might be shared memory paramters.
    Thanx once again for enlightening me.

  5. #5
    Join Date
    Jan 2001
    hi vishy

    recently i had similar experience .to change the shared memory values .there is one important point i would like to share when you alter these parameters you will not only have to bounce back the server but you will also have to RECOMPILE THE KERNEL in order for the cahnges to take place .for this depending upon your o/s you could use sam on hp-unix linuxconf on redhat linux etc...........


  6. #6
    Join Date
    Feb 2001
    Thanks hrishy,
    I did exactly the same and the results were successful.
    Thanks to all the people,

  7. #7
    Join Date
    Jun 2000
    Iselin, New Jersey, USA


    Hi Hrish

    R u shure that changing /etc/system parameters require rebuildof the kernel ? I feel just a boot is enough

  8. #8
    Join Date
    Feb 2001
    Changing /etc/system on sun does not need a recompilation of the kernel. For some flavors of linux and unix it is a needed step. Please check OS Specific docs during the install.


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