Hypervisor Memory

[KPKIND]

Hi All,

I have a P570 system where there is 64G mem and 16 proc installed. The effective memory that I can use after taking out hypervisor memory seems to be 57G. Now I am adding another 16G, so was trying to work out how much memory will I be able to use effectively after this upgrade. The quickest way would be to install and see once the system comes up but is there a quick formula kind of thing which gives you an approximate figure. I know the whole hypervisor memory depends on no. of partitions, memory allocations and virutal devices etc.. but is there a quick way of finding how much mem will I be able to use taking all these in to considerations.

BTW I have used the SPT/LVT by IBM which ridiculously gives that I can use 77G, whereas the fact is that out of 64G I have already lost 7G...which shows that SPT doesn't give you real time data.

Only thing I haven't considered while using SPT is I have not included any VIO devices, but rest of the CPU & mem allocations are all realtime data.

ANy help is very much appreciated

TIA
Kumar

 

[p5wizard]

There is no quick formula... it depends on the number of LPARs, the size (in GB) of the LPARs, the number of VLANs you're building...

If you're just adding memory to give current LPARs more room, then your memory overhead will not grow - at least not a lot.

Remember that every LPAR needs to build it's own memory page table and that is a static table, built to accomodate the LPAR's max memory setting. That's a reason not to oversize each LPAR's max mem setting! So if you don't add LPARs and don't modify the max mem setting of the existing LPARs, then chances are you'll have all the memory you're adding as available to deal around.

ROT + own experience: expect to loose about 10% of your memory for the server's own usage. You're currently a bit over that, so I'd estimate on the high side of 10%. (8 to 9 GB mem overhead for a 80GB server)

Anyway, you could get more out of your server if you go up 128Gb or more, (another ROT: 8GB or more per CPU). You can keep your CPU's occupied if you can make enough LPARs with enough RAM and then get the full benefit of virtualized CPU usage.



HTH,

p5wizard

 

[tempest92]

i found this link which gives the page table space -

http://www.ibm.com/developerworks/aix/library/au-aixpartition/index.html?ca=drs-#fig07

 

[KPKIND]

Thanks guys for your replies. I already thought there would be no straight formula as the dependency being on variable factors like number of lpars, max size settings & vlans...

Anyway Thanks for your inputs

Cheers
Kumar

 

[oliverbeat]

Hi
on the power5 there i no more the possibility to do the calculation of how memory the hypervisor used... On power4 we could do it but no more on power5. But IBM said on power 6 we still can't do it but the hypervisor will use less memory than p5... will see then.
I am not helping but ..