LPAR Memory Overhead

Here’s a simple thing that I ran across. I have a vendor that recommended that I set the Maximum memory in my LPARs to the system maximum. That way you never have to reboot to increase the maximum memory in that LPAR. I found out later that setting your LPARs memory to the system maximum makes the hypervisor allocate more memory for overhead.

This is a very old configuration issue, but I just ran across the actual numbers. When the LPAR is activated, the hypervisor allocates 1/64th the LPAR maximum for page frame tables. This is a memory structure that the hypervisor uses to track the memory pages used by the LPAR. So, lets say you have a 128GB managed system with LPARs that only really need 16GB of RAM, but the LPAR’s maximum memory is set to 128GB. By the time you’ve activated your 7th LPAR your using 2GB per LPAR, or 14GB of RAM, just for the hypervisor memory page frame tables.

3 Comments

    Peter Zutenis

    Time for a new vendor methinks ! Thats a trap for young players – always amazes me how many people make this basic error. Never, ever set every lpar max mem to the same as the system memory.

    What if the system you’re working on has 8TB of real memory ?

    Cheers

    tty7

    good catch. so much for vendors knowing the underpinnings.

    Pete Lancashire

    Where it hits is for example you have 4 LPARs where most of the time they all use 15 GB, but one (and only one) can need 30 GB for a couple hours.

    So you profile each to to a maximum of 30 GB, and from a script move or add memory to the LPAR that will soon become a hog, let is do its thing and then
    remove the memory.

    The issue is the system will reserve for 120 GB even though you will never
    use more then 75 GB ( 15 x 4 + 15)

    That’s where I’m at, and it is not 4 that at times,but 12 LPARs and only two of those 12 need the extra memory from time to time.

    I’m about 1/2 way though configuring and I’ve already “lost” 50 GB

    Have you priced what a GB of RAM is in a p770 ? .. ouch

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