How to calculate Processor Affinity Mask for an Application Pool using Powershell?

Question

I would like to be able to select which CPUs my IIS app pool is using. It is possible to set an Affinity Mask in the pool settings but I would like to be able to calculate the mask programmatically depending the available cores. For example run on all available cores except 1 and 2.

I wrote two helpers which convert the mask to a list of cpu numbers and a list of cpu numbers to a mask but it feels like I should be using bitwise operators. I am curious how you would do it, is there an easier way or if there are built in functions or modules I can use to achieve the same.

https://serverfault.com/questions/471105/formula-for-processor-affinity-mask-iis

My example:

function Get-Cpus($mask){

    $binary = [Convert]::ToString($mask,2).ToCharArray()
    $i = $binary.Length - 1
    $cpus = @()

    for(;$i -ge 0; $i--){
        if($binary[$i] -eq "1"){
            $cpus += $binary.Length - $i - 1
        }
    }

    return $cpus | Sort-Object
}

function Get-Mask($cpus){
    $cpus = $cpus | Sort-Object
    $i = $cpus[$cpus.Length -1]
    $binary = ""

    for(; $i -ge 0; $i--){
        if($cpus -contains $i){
            $binary += "1"
        }
        else{
            $binary += "0"
        }
    }

    return [Convert]::ToInt64($binary, 2)
} 

I tested my helpers by converting back and forth and checking if I get the same values.

[string]::Join(",", (Get-Cpus ([Convert]::ToInt64("00000000111111111111000000000000", 2))))
[string]::Join(",", (Get-Cpus 4294967295))
[string]::Join(",", (Get-Cpus (Get-Mask 2,9,5,23,4,7,31)))
[string]::Join(",", (Get-Cpus (Get-Mask 2,4,5,7,9,23,31)))
[string]::Join(",", (Get-Cpus (Get-Mask 1,2,30,31)))
[string]::Join(",", (Get-Cpus (Get-Mask 30,2,1,31)))
[string]::Join(",", (Get-Cpus (Get-Mask @(0..31)))) 

Answer 1

Well you can simplify Get-Mask without bitwise ops:

function Get-Mask($cpus) {
    $cpus | Foreach -Begin {$mask=0} -Process {$mask += [Math]::Pow(2,$_)} -End {$mask}
}

In Get-Cpus you could use a shift-right to simplify. Normally I would just divide by 2 but PowerShell well return doubles for non-integer results.

function Get-Cpus($mask) {
    for ($i=0; $mask -gt 0; $i++) {
        if ($mask % 2 -eq 1) {
            $i
        }
        $mask = $mask -shr 1
    }
}

Answer 2

The "classic" way would be like this:

function Get-CPUs { 
param([int64]$mask)
  $a=@()
  $i=0
  while ($mask -gt 0) {
    if ($mask -band 1 -gt 0) { $a+=$i}
    $i+=1
    $mask=$mask -shr 1
  } 
  return $a
}

function Get-Mask {
param ([int64[]] $cpus)
  $mask=0
  foreach ($cpu in $cpus) { $mask = $mask -bor (1 -shl $cpu)}
  return $mask
}

Instead of using [Math]::pow(2,x) one can just do real left shift of 1, up to 64. Technically it'll be better that if there would be a 64 or more in $cpus , an exception will be thrown.

"Easier" ways consist of the same operation, just compacted into a function somewhere.