how low can memory go? - Aix

This is a discussion on how low can memory go? - Aix ; True or false? If REAL memory is low - the OS swaps/pages/uses virtual memory. If Virtual memory is low - nothing happens. If both are low - (what is low enough?) - the OS stops 'doing stuff' ie allowing new ...

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Thread: how low can memory go?

  1. how low can memory go?

    True or false?

    If REAL memory is low - the OS swaps/pages/uses virtual memory.

    If Virtual memory is low - nothing happens.

    If both are low - (what is low enough?) - the OS stops 'doing stuff'
    ie allowing new processes ... and you pretty much have to smack the
    button for a reboot.

  2. Re: how low can memory go?

    On 2007-12-28, adnog@yahoo.com wrote:

    > True or false?
    >
    > If REAL memory is low - the OS swaps/pages/uses virtual memory.
    >
    > If Virtual memory is low - nothing happens.
    >
    > If both are low - (what is low enough?) - the OS stops 'doing stuff'
    > ie allowing new processes ... and you pretty much have to smack the
    > button for a reboot.


    Every process gets its own virtual address space from which it can allocate
    memory. In that sense, no process will ever use "real" memory. The Operating
    System will manage which parts of the virtual address space of each process
    will be in actual RAM and which parts will be in paging space on disk at any
    given time. A process could have allocated, say, 1 gigabyte of memory from
    its virtual address space, filled it with data right after allocation, and
    didn't read or write from it after that. Chances are that the OS decides
    to page out that 1 gigabyte to paging space. When the process then
    accesses (a part of) that memory, the OS has to page in the accessed memory
    from disk to actual RAM.

    You run into problems when (simply put) all *active* memory from all processes
    together doesn't fit in actual RAM. When that happens, the OS has to start
    paging madly, and if the situation doesn't improve your OS will effectively
    grind to a halt.

    --
    Jurjen Oskam

    Savage's Law of Expediency:
    You want it bad, you'll get it bad.

  3. Re: how low can memory go?

    adnog@yahoo.com wrote:
    > True or false?
    >
    > If REAL memory is low - the OS swaps/pages/uses virtual memory.
    >
    > If Virtual memory is low - nothing happens.
    >
    > If both are low - (what is low enough?) - the OS stops 'doing stuff'
    > ie allowing new processes ... and you pretty much have to smack the
    > button for a reboot.


    The overall effect is that nothing much happens, but underneath the
    system is frantically swapping pages in/out to accommodate all the
    activity. Read up on "thrashing".

    A system that is low on both virtual and real memory will become
    unresponsive since new processes may require more than the available
    free memory to get spawned and existing processes do not get execution
    time due to the thrashing state.

    A user may think it prudent to define a really large paging space, but
    the I/O overhead that results from paging between memory and disk on a
    highly active system may be so significant that the server grinds to a
    standstill even _before_ running out of paging space.

    Niel

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