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from IBM redbook of "Linux Performance and Tuning Guidelines" As we stated before, when page reclaiming occurs, the candidate pages in the inactive list which belong to the process address ...
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  1. #1
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    when Memory bottleneck


    from IBM redbook of "Linux Performance and Tuning Guidelines"
    As we stated before, when page reclaiming occurs, the candidate pages in the inactive list which belong to the process address space may be paged out. Having swap itself is not problematic situation.
    While swap is nothing more than a guarantee in case of over allocation of main memory in other operating systems, Linux uses swap space far more efficiently.
    Virtual memory is composed of both physical memory and the disk subsystem or the swap partition. If the virtual memory manager in Linux realizes that a memory page has been allocated but not used for a significant amount of time, it moves
    this memory page to swap space.

    Often you will see daemons such as getty that will be launched when the system starts up but will hardly ever be used. It appears that it would be more efficient to free the expensive main memory of such a page and move the memory page to swap.

    This is exactly how Linux handles swap, so there is no need to be alarmed if you find the swap partition filled to 50%.

    The fact that swap space is being used does not indicate a memory bottleneck; instead it proves how efficiently Linux handles system resources.
    so how can I know if the Installed Memory is insufficient ?
    I know that Linux systems utilized/grab the physical memory(in advance), thats why the value of "Used" in "free -m" is always too high. thats why I always depend upon the swap usage .. i.e if swap usage is 0 or below 20%, I think that there is no memory bottleneck .. or memory is sufficient.

    But as quoted above, even 50% usage of swap is not bad... then how can I come to know if the installed memory is insufficient ?

    I mean is there any command and value/figure that shows that installed Physical Memory is sufficient or insufficient ?

    Regards
    needee

  2. #2
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    The way I determine if there isn't enough memory is by looking at the amount of swap activity. Not how much of the swap file that's been used. I do this with the vmstat utility. If I often see the swap so (Amount of memory swapped to disk) column greater than zero then that's an indication there's not enough memory. It shows that the OS often has to make room in memory for something that wants to run.

  3. #3
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    Quote Originally Posted by vsemaska View Post
    The way I determine if there isn't enough memory is by looking at the amount of swap activity. Not how much of the swap file that's been used. I do this with the vmstat utility. If I often see the swap so (Amount of memory swapped to disk) column greater than zero then that's an indication there's not enough memory. It shows that the OS often has to make room in memory for something that wants to run.
    Hi thanks for help.
    OK I have two types of Linux Server
    (a) File Servers(NFS/SAMBA) with 4 GB memory
    (b) database Servers(Oracle/Sybase) with 16 GB memory

    then which value of "so" columns in vmstat, is acceptable.. or what value of "so" indicates that I have to buy a new Physical Memory.
    or just greater then zero is bad ? if the value of "so" is in b/w 20 and 70, is it bad ?

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  5. #4
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    Ideally the 'so' column should always be zero. Whether a non-zero count is bad depends on if response is slow when the server is busy. If there is a non-zero count on your database servers then you could try changing cache sizes. For Oracle databases you could adjust SGA & PGA values.

  6. #5
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    Quote Originally Posted by vsemaska View Post
    Ideally the 'so' column should always be zero. Whether a non-zero count is bad depends on if response is slow when the server is busy. If there is a non-zero count on your database servers then you could try changing cache sizes. For Oracle databases you could adjust SGA & PGA values.
    thanks for explaining
    Regards
    needee

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