error in vCPU calculation documentation

[egernst]

See https://github.com/kata-containers/documentation/blob/master/design/cpu-constraints.md

The formula is listed as:

(quota + (period -1)) / period

This doesn't match the code, and it is not correct. vCPU = quota / period

This is related to kata-containers/runtime#1767

[egernst]

/cc @devimc @jcvenegas

[shsha-msft]

Just to clarify. Is the following understanding correct?

For containers in a POD the formula is vCPU = quota / period
For the POD itself the formula is : (quota + (period -1)) / period

/cc @gkhanna79

[jcvenegas]

The calculation is as follow:

The containers in a pod are converted to milli cpus

func CalculateMilliCPUs(quota int64, period uint64) uint32 {

        // If quota is -1, it means the CPU resource request is
        // unconstrained.  In that case, we don't currently assign
        // additional CPUs.
        if quota >= 0 && period != 0 {
                return uint32((uint64(quota) * 1000) / period)
        }

        return 0
}

Example:

//  This is 1500
milliCPUs:= CalculateMilliCPUs(150000, 100000)

When a pod has mutiples containers the milliCPUs are sum:

millCPUs := 0
milliCPUs += CalculateMilliCPUs(150000, 100000)
milliCPUs += CalculateMilliCPUs(150000, 100000)
// This result in 3000 millcpus

After the milliCPUs for a POD are calculated this is converted to CPUs with:

//CalculateVCpusFromMilliCpus converts from mCPU to CPU, taking the ceiling
// value when necessary
func CalculateVCpusFromMilliCpus(mCPU uint32) uint32 {
       return (mCPU + 999) / 1000
}

VCPUS:
(3000 + 999 ) / 1000 = 3

[egernst]

@jcvenegas overall, though, the documentation is wrong.

It should be vCPU = ceiling( quota / period )

Can you send a PR to update the doc? WDYT?