PowerVM

Is it possible to dynamically reduce the reserved memory for Hypervisor? I have a system that was using VIOs that has since been rebuilt as a standalone full system resource partition. Reserved memory is showing as several Gb and could be needed in this environment due to workload, lack of live partition mobility and business outages (so partition can not be moved with an outage).

The previous VIO partitions are still there but not activated. Could removing these reduce reserved hypervisor memory?#

Thanks

Hi!

Reserved memory for hypervisor is not directly influenced by the presence of VIO servers, as they use normal RAM.

In my experience things that contribute to hypervisor RAM usage is the RAM and CPU scalability of each partition.

Lets say you have a partition using 10 vCPU (Desired) and Max is 30, then hypervisor RAM has to be allocated for the dynamic scalability ot 30 vCPU.

This is especially noticable for RAM. So if your application requires 100GB RAM (Desired) but you have given it 200 GB Max, apx 4% of that 100GB is reserved for the page table entries by the hyervisor.

Hypevisor Mirroring if enabled  also contributes to RAM usage.

SR_IOV Network adapters reserve RAM.

If this is an older server, Disable Hypervisor IO adapter enlarged capacity, as descibed on page 59 here:

https://ibm.ent.box.com/v/powerbestpractice

So the most effective way to cull hypervisor RAM usage is to keep MAX and DESIRED values of RM as close as possible, without sacrificing application availability.

------------------------------
FREDRIK LUNDHOLM

Original Message:
Sent: Tue March 07, 2023 09:14 AM
From: Dave Heald
Subject: PowerVM Hypervisor memory

Is it possible to dynamically reduce the reserved memory for Hypervisor? I have a system that was using VIOs that has since been rebuilt as a standalone full system resource partition. Reserved memory is showing as several Gb and could be needed in this environment due to workload, lack of live partition mobility and business outages (so partition can not be moved with an outage).

The previous VIO partitions are still there but not activated. Could removing these reduce reserved hypervisor memory?#

Thanks

goodday,

Some more info on the PageTable calculation:

In the ASMI interface you can set the LMB size (Logical Memory Block). If this is set at 256MB you can increase lpar memory in steps of 256MB. This LMB size is also being used for the calculation of the AIX PageTable (each AIX lpar has a PageTable). This size is calculated by dividing the max memory setting in your profile , divided by the LMB size. So if you have max 64GB , then 64GB / 256MB = 256MB AIX Page Table which is rather large and most probably overkill. This AIX PageTable is take from real memory and in my believe is part of the Hypervisor memory. So if you have 10 lpar with each 256MB pagetable , then hyporvisor memory is 10 x 256MB.

Your other remarks makes also perfectly sense!

Regards

Michel.

Hi!

Reserved memory for hypervisor is not directly influenced by the presence of VIO servers, as they use normal RAM.

In my experience things that contribute to hypervisor RAM usage is the RAM and CPU scalability of each partition.

Lets say you have a partition using 10 vCPU (Desired) and Max is 30, then hypervisor RAM has to be allocated for the dynamic scalability ot 30 vCPU.

This is especially noticable for RAM. So if your application requires 100GB RAM (Desired) but you have given it 200 GB Max, apx 4% of that 100GB is reserved for the page table entries by the hyervisor.

Hypevisor Mirroring if enabled  also contributes to RAM usage.

SR_IOV Network adapters reserve RAM.

If this is an older server, Disable Hypervisor IO adapter enlarged capacity, as descibed on page 59 here:

https://ibm.ent.box.com/v/powerbestpractice

So the most effective way to cull hypervisor RAM usage is to keep MAX and DESIRED values of RM as close as possible, without sacrificing application availability.

------------------------------
FREDRIK LUNDHOLM

Original Message:
Sent: Tue March 07, 2023 09:14 AM
From: Dave Heald
Subject: PowerVM Hypervisor memory

Is it possible to dynamically reduce the reserved memory for Hypervisor? I have a system that was using VIOs that has since been rebuilt as a standalone full system resource partition. Reserved memory is showing as several Gb and could be needed in this environment due to workload, lack of live partition mobility and business outages (so partition can not be moved with an outage).

The previous VIO partitions are still there but not activated. Could removing these reduce reserved hypervisor memory?#

Thanks

Many years ago I was told that the size of a hypervisor page table for an LPAR is always rounded up to a power of two.  Example: an LPAR with a maximum memory of 33GB has the same size HPT as an LPAR with a maximum memory of 64GB.  The moral: You should always set the maximum memory to an even power of two.

Is this correct?  Was it ever correct?

goodday,

Some more info on the PageTable calculation:

In the ASMI interface you can set the LMB size (Logical Memory Block). If this is set at 256MB you can increase lpar memory in steps of 256MB. This LMB size is also being used for the calculation of the AIX PageTable (each AIX lpar has a PageTable). This size is calculated by dividing the max memory setting in your profile , divided by the LMB size. So if you have max 64GB , then 64GB / 256MB = 256MB AIX Page Table which is rather large and most probably overkill. This AIX PageTable is take from real memory and in my believe is part of the Hypervisor memory. So if you have 10 lpar with each 256MB pagetable , then hyporvisor memory is 10 x 256MB.

Your other remarks makes also perfectly sense!

Regards

Michel.

Hi!

Reserved memory for hypervisor is not directly influenced by the presence of VIO servers, as they use normal RAM.

In my experience things that contribute to hypervisor RAM usage is the RAM and CPU scalability of each partition.

Lets say you have a p