Originally posted by: da_di

Hallo,

we like to set the number of MAXIMUM number virtual processor(=16) of each DLPAR(=5) in a P5-570 to the number of TOTAL processor(=16) in the processor pool.
In this situation we are able to set the number of current virtual processor via HMC for each DLPAR according the workload situation(developing environment).
Our provider gives us the information, that these settings will result in performance impact or loosing CPU-Power for DLPAR-Overhead.
i agree, that setting CURRENTLYDESIRED number of virt. Procossor to 16 for each DLPAR could be a Problem. But MAxIMUM Number?

thank you very much for any help

best regards

W. Grumptmann
#AIX-Forum

Originally posted by: nh4aix

Hi -

my experience as been that setting MAXIMUM to total processors for each LPAR has some, but very little overhead. What seems to cause a ton of overhead is setting MAXIMUM on memory equal to the total memory on the server for each LPAR!!! That may be what your provider was indicating...

The reasoning is that for value in MAXIMUM (memory or CPU), the hypervisor has to pre-allocate that much memory to handle that resource, just in case. In the case of CPU, you are talking 16 * 5 or 80 pre-allocated memory blocks (or whatever they are called). For memory, because memory is allocated in 4K pages (I know AIX is in that increment.. the hypervisor may be a lower number, but the illustration will work regardless), and assuming you have 16GB of memory and you allocate 16GB of memory as MAXIMUM for each of your 5 LPARs, that is 16GB / 4KB * 5 or 20Million pre-allocated memory blocks...

Well that is my understanding... so I don't worry about setting MAXIMUM for processors too high and I try to keep my MAXIMUM for memory equal to DESIRED + 10% to give me some wiggle room with a minimal amount of overhead...

hope that helps...

Joe
#AIX-Forum

Originally posted by: SystemAdmin

The number of virtual processors is used by AIX to dispatch the corresponding number of threads per timeslice. So if the count is 16 that means 16 threads will be dispatched at a time (assuming that there are those many threads requiring CPU time). If your backend shared pool does not have the that much of processing power it leads to threads waiting for the next cycle (though they did get dispatched). If your pool does have that much of processing power, each thread gets very little CPU time per timeslice to do whatever it has to do. If you do not have so many threads to actively use the CPU at one go, you may not see any drop in performance if VP is at max.

What I generally do is for 1.6 processing units, I set VP as 2. Then under normal workload, I keep increasing this by 1 and monitoring the performance. At first you will see some gain in performance until a certain point and then it decreases (Like a bell curve). Thats around where you need to stop.

Hope that helps.

r/
R
#AIX-Forum

Originally posted by: SystemAdmin

Oh and having higher VP is more beneficial for uncapped lpars than capped.

r/
R
#AIX-Forum

Originally posted by: da_di

thank you very much for you answer,

let me ask the question in an other way:

there are 3 Values in the HMC Settings for defining virt. processor to a DLPA belonging to a proc.pool on a Server:

• Minimum virtual Processor
• Desired virtual Procossor
• Maximum virtual Processor

in our case, 16 processor belongs to the proc.pool for the DLPARS

we like to do following setting:

• Minimum virtual processor = 1 for each DLPAR, in SUMall DLPARs < 16
• Desired virtual processor = 16 in sumall DLPARs
• Maximum virtual processor = numberall lpars x 16

so, each DLPAR can set to maximum processorusage dynamically (we are developing, so workload balancing is a challange for the administrator)

do you know, if the are any performance impact due to this high maximum setting?

thank you very much

kind reguards
w.grumptmann
#AIX-Forum

Originally posted by: SystemAdmin

I cant think of any other performance issues than what I have already listed.

Remember at lpar startup whatever VP is set as desired value would be assigned to the lpar. Any changes after that would be via dlpar and within the min max boundaries.

Ultimately it would be your call as to how many VP's would suffice before performance starts degrading.

r/
R
#AIX-Forum

Originally posted by: SystemAdmin

Aslo just NOTE each Virtual Processor Consumes 0.1 CPU. so in your case if you set 16 virt. Proc. It may take 1.6 CPU , correct me guys if it's different ?
#AIX-Forum

Originally posted by: SystemAdmin

I do not think thats right. If I create an LPAR with 0.1 processing units then the min VP we can assign is 1, does that mean the LPAR cannot be used? I have such lpars running in our environment.

r/
R
#AIX-Forum

Originally posted by: nh4aix

in response to: "Aslo just NOTE each Virtual Processor Consumes 0.1 CPU. so in your case if you set 16 virt. Proc. It may take 1.6 CPU , correct me guys if it's different ?"

This is not my understanding... the MINIMUM amount of CPU you can define is 0.1, but if the VP is ideal, it consumes zero. If it is busy, in can consume in increments of 0.01...

So if you set 16 VP and the LPAR is idle, it will consume zero.

joe
#AIX-Forum

Originally posted by: da_di

>> So if you set 16 VP and the LPAR is idle, it will consume zero. <<

That is also my understanding.

Thank you all for your participation

W.Grumptmann
#AIX-Forum