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# CPU usage

Posted on 2013-06-12
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Hi -

We have a IBM AIX server using LPAR. This has 9 physical cpus (entitled) and 80 logical cpus. The logical CPU % of Utilization is never matching with the physical cpu utilization. When Physical CPU maxed out the logical CPU still showing only 30% of usage.

Please find the attached excel sheet for more details.

IF you look at the LPAR tab in the given excel, when the entitled cpu usage maxed out, the logical and physical should report the same % of CPU utilization.

Thanks!

Sve.
Book15.zip
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Question by:sventhan
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Expert Comment

ID: 39242776
uname -r
And explain what problem you are trying to solve.

physical CPU is hared between virtual CPUs, and virtual one is waiting for IO while rest of
physical CPU power is used by other virtual CPUs. the numbers will never add up

Please unzip document yourself, i am not good at peeling onions.
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Author Comment

ID: 39242927
Thanks for your time. I've peeled the onion for you.

The Question is,

How 1 Physical CPU match with the X logical CPU?  How this is getting configured?

When all the CPUs being used , PCPU = 100% why lcpu is only 30%?

What is the ratio between these 2 cpus?

Thanks,
sve.
Book15.xlsx
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LVL 62

Accepted Solution

gheist earned 500 total points
ID: 39243096
100/30 - ratio is 333,3%

Pardon - but what PROBLEM you are trying to solve?

100% PCPU = 100% LCPU
LCPU usage can be less because:
VCPU is guaranteed num_pcpus/num_vcpus of cpu time (in your case - 9/80=0.113=11,3% of a single CPU)
If one does nothing (or waits for slow network IO) other VCPUs can use spare time
If enough of them do nothing system can save power by halting and slowing down unused PCPUs.

Remember that not all workloads are calculations on CPU.
Database would wait a lot for disk IO
While webserver will wait for network io
Or websphere will spend huge amounts of time blocking and synchronizing threads
All three showing as "wait" and/or "idle" in topas
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Author Closing Comment

ID: 39245641
Thanks
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