Shmmax??
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Thread: Shmmax??

  1. #1
    Join Date
    Nov 2004
    Location
    Mumbai, India
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    452

    Shmmax??

    Well one of the Know-it-all DBA asked me this question (He was trying to test me )

    RAM : 8 G.B
    SHMMAX : 2. G.B
    SGA (Shared Pool ,Cache...etc) : 4 G.B
    Will the instance start ??

    I said:
    Yes. Two Segments of 2 G.B each will be created.
    He Said :
    No. You are wrong. The instance won't start.

    Am I wrong??
    There are three kinds of lies: Lies, damned lies, and benchmarks...

    Unix is user friendly. It's just very particular about who it's friends are.

    Oracle DBA

  2. #2
    Join Date
    Apr 2001
    Posts
    110
    Hi,

    Oracle recommends setting SHMMAX to 4Gb or 0.5 x Physical Memory on the system.

    All our systems have SHMMAX set to 4Gb and our SGA are less that 4Gb.

    However, I'm not sure if your system will actually create 2 shared memory segments for this case.

    Cheers
    Oracbase
    What's next after 10g?

  3. #3
    Join Date
    Jan 2001
    Posts
    2,828
    Hi SimplyDBA

    An application requests a given amount of contiguous shared memory from the OS.
    The OS dictates how large of a shared memory segment it will allow with the kernel parameter SHMMAX(Shared Memory Maximum).
    If the amount of shared memory requested by the application is greater than SHMMAX, the OS may be granted the shared memory in multiple segments.

    Ideally, however, you want the amount requested by the application to be less than SHMMAX so that the application's request can be fulfilled with one shared memory segment.

    So bottomline simply dba ur right .

    Just found a metalink note 15566.1 (if that gives a boost to your ego and inflates it to infinity :-D)

    regards
    Hrishy
    Last edited by hrishy; 06-08-2005 at 09:21 AM.

  4. #4
    Join Date
    Nov 2004
    Location
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    Thanks hrishy !
    Too bad i could not see the note .Metalink shows its down (although the date and time is old)
    There are three kinds of lies: Lies, damned lies, and benchmarks...

    Unix is user friendly. It's just very particular about who it's friends are.

    Oracle DBA

  5. #5
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    Sep 2002
    Location
    England
    Posts
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  6. #6
    Join Date
    Nov 2004
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    I know. It was my browser's cache problem
    There are three kinds of lies: Lies, damned lies, and benchmarks...

    Unix is user friendly. It's just very particular about who it's friends are.

    Oracle DBA

  7. #7
    Join Date
    Nov 2004
    Location
    Mumbai, India
    Posts
    452
    Ah. The doc as really gave some enlightenment
    Just one doubt
    With mutliple segements there are two possiblities.The segments can be attached contiguously, so that it appears to one large shared memory segment, or non-contiguously, with gaps between the segments.The formaer wastes less space that could be used for the stack or heap, but depending on alignment requirements for the shared memory (defined by SHMLBA in the kernel), it may not be possible
    What is meant by the statement (in italics)?
    There are three kinds of lies: Lies, damned lies, and benchmarks...

    Unix is user friendly. It's just very particular about who it's friends are.

    Oracle DBA

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