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Let us assume that I work in a company and this company possesses just one server. At the end of every month my boss enters my office and asks me: 'Do we need new CPUs?'. Let us also say that, as a rule of thumb, the company needs new CPUs if more than 70% of the 'systems resources' are used. When typing lscpu I get

fabi@alien-linux:~$ lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                8
On-line CPU(s) list:   0-7
Thread(s) per core:    2
Core(s) per socket:    4
Socket(s):             1
...

So I compute

X = (Nr of CPUs)*(Nr of sockets)*(Nr of cores per socket)*(Threads per core)

which is 84 in my case. For simplicity, lets assume that I execute "top" at a very high sampling rate throughout the month and it constantly shows

 50.0 50.0 50.0

which means that, in average, 50 processes were either actually dealt with by the cores, waiting to be dealt with or in the state "UNINTERRUPTABLE". Fine, I compute

0.7 * 84 = 58.8

so this means '70% of the systems resources = 58.8 processes'.

Since I only have 50 processes, I can answer 'no' to my boss, we do not need new hardware.

Here is my question:

Is this correct?

The question has been asked here and the answer was that this computation is not correct. I do not understand why...

best regards,

FW

fixer1234
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Fabian Werner
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  • This is an overly simplistic set of metrics to answer the question at hand. There are other many other meaningful metrics to evaluate when determining your hardware needs. – Frank Thomas Feb 10 '15 at 12:37
  • Could you elaborate this a little bit more in detail? Just to get me started. – Fabian Werner Feb 10 '15 at 12:49
  • you have to look at your applications and your capacities. do the apps you support run optimally for your users even at peak times? how does ram usage fluctuate throughout the day, how long are you waiting for storage access, is storage growth within expectations, etc. – Frank Thomas Feb 10 '15 at 13:21
  • Yes, but all this refers to things different from CPU usage. – Fabian Werner Feb 10 '15 at 13:52
  • Nothing about the question "Do we need new Hardware?" indicates a fixation on the CPU. – Frank Thomas Feb 10 '15 at 16:44
  • Well, I thought this (i.e. the exclusion of *all* other factors that may cause a bottleneck) was clear by the nature of the question. Edited to "Do we need new CPUs?". – Fabian Werner Feb 13 '15 at 12:27

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