LPAR - Weighting? 1

[mrn]

Hi

I understand what lpar weighting is used for (with VIO), but I can't seem to get my head round what I should be setting weighting to. As far as I can make out a partition with a weight of 2 can steal resources from one with a weight of 1, and a weight of 3 can steal from 2 & 1.

I'm I correct or I'm I missing something?

Thanks in advance.

Mike

"Whenever I dwell for any length of time on my own shortcomings, they gradually begin to seem mild, harmless, rather engaging little things, not at all like the staring defects in other people's characters."

 

[p5wizard]

A p5 LPAR doesn't actually "steal" resources from other LPARs... In a p5 shared processor environment, there are usually CPU resources left over - the amount of CPU that is left after all the entitlements for all the running LPARs are deducted from the amount of CPUs that are in the box.

The shared pool gets distributed among the LPARs that are "uncapped" and have enough room to grow in their virtual CPUs - i.e. LPARs that have the ability to go beyond their entitled CPU capacity. The distribution of the free cycles in the CPU pool is calculated using the difference in the "weights" of the LPARs that want/need extra CPU.

Say there are 6 free 10ms CPU slots and 3 LPARs want it

LPARa has weight 50
LPARb has weight 100
LPARc has weight 150

so:

LPARa will get 1 extra 10ms cycles
LPARb will get 2 extra 10ms cycles
LPARc will get 3 extra 10ms cycles



HTH,

p5wizard

 

[mrn]

Ok Thanks a little clearer but in the redbook "Partitioning Implementations
for IBM p5 Servers" it states

When determining if resources can be taken from an excess user, the weight of 
the partition is determined to define the priority. Higher priority partitions can take resources from lower priority partitions. A partition's priority is defined as the ratio of its excess to its weight, where excess is expressed with the formula (current amount - desired amount) and weight is the policy-defined weight. A lower value for this ratio represents a higher priority.

Which seems to imply that it can steal if it needs to.

Mike

"Whenever I dwell for any length of time on my own shortcomings, they gradually begin to seem mild, harmless, rather engaging little things, not at all like the staring defects in other people's characters."

 

[p5wizard]

Yes, but only from an "excess" user, one which is already "over their entitlement". It's like: one of the kids has hoarded most of the LEGO bricks, so mom steps in: "OK everyone, hand them all the loose bricks back, and I'll redistribute. The distribution is then based upon their different "weights" and also how "big" the kids are.

An LPAR with one virtual CPU and 0.80 CPUs entitlement can only consume an extra 0.20 CPUS because a virtual CPU can be anywhere between 0.10 and 1.00 CPUs so even if that LPAR's weight is 255 and there's 6 free CPUs in the pool, 0.20 is all the excess-use it will get.

Also, if I'm not mistaken, LPARs with can "cede" some of their CPU cycles if they are idle enough and do not need them; thus entering those cycles into the shared pool and making it bigger for more cycles to be handed out to all excess-user LPARs.



HTH,

p5wizard

 

[mrn]

p5 - now you've found my level.....lego ;-)

Mike

"Whenever I dwell for any length of time on my own shortcomings, they gradually begin to seem mild, harmless, rather engaging little things, not at all like the staring defects in other people's characters."