log file variables in function eat up all memory

Powershell or .Net has a garbage collector, so freeing the memory isn't instant. Garbage Collection in Powershell to Speed Scripts Also the memory management is probably better in Powershell 7. I tried repeating your function many times, but the memory usage didn't go above a few hundred megs.

There's probably some more efficient .net way to prepend a line to a file: Prepend "!" to the beginning of the first line of a file

I have a weird way to prepend a line. I'm not sure how well this would work with a large file. With a 700 meg file, the Working Set memory stayed at 76 megs.

$c:file
one
two
three

$c:file = 'pre',$c:file

$c:file
pre
one
two
three

ps powershell

Handles NPM(K) PM(K) WS(K) CPU(s)   Id SI ProcessName
------- ------ ----- ----- ------   -- -- -----------
    607     27 67448 76824   1.14 3652  3 powershell

I just want to rule out the RAM usage is caused by prepending to the file. Have you tried not storing the log contents in a variable? i.e.

$logMessage,(Get-Content -Path $logFilePath) | Out-File -FilePath $logFilePath

Edit 5/8/20 - Turns out that prepending (when done efficiently) isn't as slow as I thought - it is on the same order as using -Append. However the code (that js2010 pointed to) is long and ugly, but if you really need to prepend to the file, this is the way to do it.

I modified the OP's code a bit to automatically insert a new line.

function log-prepend{
  param(
    $content,
    $filePath = 'C:\temp\myLogP.txt'
  )
    $file = get-item $filePath
    if(!$file.exists){
      write-error "$file does not exist";
      return;
    }
    $filepath = $file.fullname;
    $tmptoken = (get-location).path + "\_tmpfile" + $file.name;
    write-verbose "$tmptoken created to as buffer";
    $tfs = [System.io.file]::create($tmptoken);
    $fs = [System.IO.File]::Open($file.fullname,[System.IO.FileMode]::Open,[System.IO.FileAccess]::ReadWrite);
    try{
      $date = Get-Date -Format 'yyyyMMddHHmmss'
      $logMessage = "{0}_{1}`r`n" -f $date,$content
      $msg = $logMessage.tochararray();
      $tfs.write($msg,0,$msg.length);
      $fs.position = 0;
      $fs.copyTo($tfs);
    }
    catch{
      write-verbose $_.Exception.Message;
    }
    finally{

      $tfs.close();
      $fs.close();
      if($error.count -eq 0){
        write-verbose ("updating $filepath");
        [System.io.File]::Delete($filepath);
        [System.io.file]::Move($tmptoken,$filepath);
      }
      else{
        $error.clear();
        [System.io.file]::Delete($tmptoken);
      }
    }
}

Here was my original answer that shows how to test the timing using a stopwatch.

When you prepend to a log file, you're reading the entire log file into memory, then writing it back.

You really should be using append - that would make the script run a lot faster.

function log($Message) {
  $logFilePath = 'C:\logPath\myLog.txt'

  $date = Get-Date -Format 'yyyyMMddHHmmss'
  $logMessage = "{0}_{1}" -f $date,$Message
  $logMessage | Out-File -FilePath $logFilePath -Append
}

Edit: To convince you that prepending to a log file is a bad idea, here's a test you can do on your own system:

function logAppend($Message) {
  $logFilePath = 'C:\temp\myLogA.txt'

  $date = Get-Date -Format 'yyyyMMddHHmmss'
  $logMessage = "{0}_{1}" -f $date,$Message
  $logMessage | Out-File -FilePath $logFilePath -Append
}

function logPrepend($Message) {
  $logFilePath = 'C:\temp\myLogP.txt'

  $date = Get-Date -Format 'yyyyMMddHHmmss'
  $logMessage = "{0}_{1}" -f $date,$Message
  if(Test-Path -Path $logFilePath) {
    $logFileContent = Get-Content -Path $logFilePath
  } else {
    $logFileContent = ''
  }
  $logMessage,$logFileContent | Out-File -FilePath $logFilePath 
}

$processes = Get-Process

$stopwatch =  [system.diagnostics.stopwatch]::StartNew()
foreach ($p in $processes)
{
  logAppend($p.ProcessName)
}

$stopwatch.Stop()
$stopwatch.Elapsed


$stopwatch =  [system.diagnostics.stopwatch]::StartNew()
foreach ($p in $processes)
{
  logPrepend($p.ProcessName)
}

$stopwatch.Stop()
$stopwatch.Elapsed

I've run this several times until I got a few thousand lines in the log file. Going from: 1603 to 1925 lines, my results were:

Append: 7.0167008 s Prepend: 21.7046793 s

Although as commented, I can hardly believe that this function gobbles up your memory, you could optimize it:

function log ([string]$Message) {
    $logFilePath = 'C:\logPath\myLog.txt'
    # prefix the message with the current date
    $Message = "{0:yyyyMMddHHmmss}_{1}" -f (Get-Date), $Message
    if (Test-Path -Path $logFilePath -PathType Leaf) {
        # newest log entry on top: append current content
        $Message = "{0}`r`n{1}" -f $Message, (Get-Content -Path $logFilePath -Raw)
    }

    Set-Content -Path $logFilePath -Value $Message
}