Why is the speedup of this parallelization of CPU-bound runspaces limited to ~3x? #6965
Description
I'm experimenting with running PowerShell unit tests in parallel. The execution of most tests in my corpus is CPU-bound.
Steps to reproduce
Run the following on a computer with 8 or more cores that are otherwise idle.
$processorCount = [System.Environment]::ProcessorCount
Write-Host "Processor Count: $processorCount"
$sb = {
function fibonacci {
param([int]$n)
[bigint]$a=0
[bigint]$b=1
foreach ($x in 0..$n)
{
$a,$b = $b,($a+$b)
}
$b
}
fibonacci 100000
}
$t_sync = Measure-Command $sb
$rsp = [runspacefactory]::CreateRunspacePool(1,$processorCount)
$rsp.Open()
foreach ( $n in 1..$processorCount )
{
$ps = 1..$n | % { [powershell]::Create().AddScript($sb) }
$ps | % { $_.RunspacePool = $rsp }
$t_begin = Measure-Command {
$invocation = $ps.BeginInvoke()
}
$t_wait = Measure-Command {
while ( $invocation.IsCompleted -contains $false )
{
sleep 0.1
}
}
[pscustomobject]@{
'n ' = $n
'BeginInvoke() (ms)' = [int]$t_begin.TotalMilliseconds
'Wait (ms)' = [int]$t_wait.TotalMilliseconds
'speedup' = (($t_sync.TotalMilliseconds*$n)/$t_wait.TotalMilliseconds) | % { [math]::Round($_,3) }
}
}Expected behavior
I expected the speedup and processor utilization to be somewhat proportional to the number of runspaces as long as the number of runspaces is fewer than the number of cores.
Actual behavior
The actual speedup seems limited to ~3x even on a computer with 16 cores. As the number of runspaces nears the number of cores, the actual processor utilization seems to peak only momentarily and settles to approximately 50%.
Here are the results of a typical run:
Processor Count: 16
n BeginInvoke() (ms) Wait (ms) speedup
-- ------------------ --------- -------
1 1 2910 1.162
2 15 2990 2.263
3 15 3655 2.776
4 38 6884 1.965
5 38 8144 2.077
6 38 8450 2.402
7 44 9712 2.438
8 53 11435 2.366
9 53 14680 2.074
10 47 12754 2.652
11 61 19348 1.923
12 78 21901 1.853
13 92 23640 1.86
14 76 22776 2.079
15 85 27332 1.856
16 136 29318 1.846
Here is the CPU utilization graph from the end of the above test run. The peak occurred as the n=16 run began. The peak and plateau shape seems to be characteristic for all n.
Environment data
> $PSVersionTable
Name Value
---- -----
PSVersion 6.1.0-preview.691
PSEdition Core
GitCommitId v6.1.0-preview.691
OS Microsoft Windows 6.3.9600
Platform Win32NT
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
WSManStackVersion 3.0