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https://github.com/RetroPortingToolKit/psxrecomp
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tools: throttle a live title's CPU to reproduce a weaker host
"Runs fine here, players report slowdown there" is not reproducible on a fast dev box, and guessing at the budget has cost us whole investigations. This throttles a RUNNING title until the reported symptom appears, so the CPU budget is read off a dial instead of argued about. Uses the Windows Job Object CPU rate control -- the same hard cap the scheduler applies to containers -- so it needs no launcher or runtime support and attaches to a live process. That matters: an observability tool that requires relaunching the session destroys the state you were trying to observe, and the reported hitch often takes minutes of play to reach. Two independent knobs, both reversible while the game runs: -CoresWorth <n> hard CPU-rate cap as cores' worth of compute (slower clock) -Cores <n> processor affinity: how many logical CPUs are available -Measure sample the current draw, change nothing -Off release the cap and restore affinity The job is NAMED, so the cap can be re-dialed from a fresh shell without re-assigning the process, and the script can exit while the cap stays on. Job reuse is probed with CreateJobObject + ERROR_ALREADY_EXISTS rather than OpenJobObject, because nesting a second job would be silently unrecoverable: nested CPU caps take the MINIMUM, so a loosened dial would appear to do nothing. Releasing a cap sets it to the full machine (CpuRate 10000) rather than clearing the control block, because the API validates the union against the flags and rejects an all-zero block with ERROR_INVALID_PARAMETER. Read it as a reproduction tool, not a profiler: the hard cap is enforced by withholding scheduling quanta, so a heavily throttled process stutters rather than degrading smoothly. Good for "does the reported hitch appear", wrong for "what is the real frame time on that CPU". The header says so too. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Matthew Stanley
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# cpu_throttle.ps1 — simulate a weaker host CPU for any psxrecomp title.
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#
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# WHY THIS EXISTS: "runs fine here, players report slowdown there" is not
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# reproducible on a fast dev box. Rather than guess, throttle the running
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# process until the reported symptom appears, then read the budget off the
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# dial. Uses the Windows Job Object CPU rate control (the same hard cap the
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# scheduler applies to containers), so it needs no launcher/runtime support
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# and works on a LIVE process — never restart a session to arm it.
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#
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# The job is NAMED, so the cap can be re-dialed at any time from a fresh shell
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# without re-assigning the process. A job object outlives the handle that made
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# it for as long as a process is still inside it, so this script exits and the
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# cap stays on.
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#
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# Two independent knobs, both reversible while the game runs:
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# -CoresWorth <n> hard CPU-rate cap, expressed as cores' worth of compute.
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# This is the "slower clock" dial. 0.5 on a 60fps title
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# that wants ~1 core is roughly a half-speed CPU.
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# -Cores <n> processor affinity: how many logical CPUs the process may
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# run on at all. This is the "fewer cores" dial.
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#
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# Usage:
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# tools\cpu_throttle.ps1 -ProcessId 51368 -CoresWorth 0.5
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# tools\cpu_throttle.ps1 -ProcessId 51368 -CoresWorth 0.35 # tighten, live
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# tools\cpu_throttle.ps1 -ProcessId 51368 -Cores 2 # 2 cores only
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# tools\cpu_throttle.ps1 -ProcessId 51368 -Measure # sample, no change
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# tools\cpu_throttle.ps1 -ProcessId 51368 -Off # remove cap + affinity
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#
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# NOTE: the hard cap is enforced by withholding scheduling quanta, so a heavily
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# throttled process stutters rather than degrading smoothly. That is fine for
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# "does the reported hitch appear", and misleading for "what is the real frame
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# time on that CPU". Read it as a reproduction tool, not a profiler.
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[CmdletBinding()]
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param(
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[Parameter(Mandatory = $true)][int]$ProcessId,
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[double]$CoresWorth = 0,
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[int]$Cores = 0,
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[switch]$Measure,
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[switch]$Off,
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[int]$SampleSeconds = 4
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)
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$ErrorActionPreference = "Stop"
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Add-Type @'
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using System;
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using System.Runtime.InteropServices;
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public static class JobCpu {
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[DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
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public static extern IntPtr CreateJobObjectW(IntPtr sa, string name);
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[DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
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public static extern IntPtr OpenJobObjectW(uint access, bool inherit, string name);
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[DllImport("kernel32.dll", SetLastError = true)]
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public static extern bool AssignProcessToJobObject(IntPtr job, IntPtr process);
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[DllImport("kernel32.dll", SetLastError = true)]
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public static extern bool SetInformationJobObject(IntPtr job, int infoClass, ref CPU_RATE info, int len);
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[DllImport("kernel32.dll", SetLastError = true)]
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public static extern IntPtr OpenProcess(uint access, bool inherit, int pid);
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[DllImport("kernel32.dll", SetLastError = true)]
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public static extern bool CloseHandle(IntPtr h);
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[StructLayout(LayoutKind.Sequential)]
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public struct CPU_RATE { public uint ControlFlags; public uint RateOrWeight; }
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// JobObjectCpuRateControlInformation
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public const int InfoClass = 15;
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public const uint ENABLE = 0x1;
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public const uint HARD_CAP = 0x4;
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// PROCESS_SET_QUOTA | PROCESS_TERMINATE | PROCESS_QUERY_INFORMATION
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public const uint PROC_ACCESS = 0x0100 | 0x0001 | 0x0400;
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public const uint JOB_ALL_ACCESS = 0x1F001F;
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}
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'@
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$proc = Get-Process -Id $ProcessId -ErrorAction SilentlyContinue
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if (-not $proc) { throw "no process with PID $ProcessId" }
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$nCpu = [Environment]::ProcessorCount
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function Measure-Draw([System.Diagnostics.Process]$p, [int]$secs) {
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$t1 = $p.TotalProcessorTime
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Start-Sleep -Seconds $secs
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$p.Refresh()
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$ms = ($p.TotalProcessorTime - $t1).TotalMilliseconds
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[pscustomobject]@{
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CoresWorth = $ms / ($secs * 1000.0)
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SystemPct = $ms / ($secs * 1000.0 * $nCpu) * 100
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}
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}
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Write-Host "process : $($proc.ProcessName) PID $ProcessId ($($proc.Threads.Count) threads)"
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Write-Host "machine : $nCpu logical processors"
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if ($Measure) {
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$d = Measure-Draw $proc $SampleSeconds
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Write-Host ("draw : {0:N2} cores' worth ({1:N2}% of machine)" -f $d.CoresWorth, $d.SystemPct)
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return
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}
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$jobName = "psxrecomp_throttle_$ProcessId"
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# Reuse the existing job if this PID was already throttled, so re-dialing does
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# not nest a second job (nested caps only ever ratchet DOWN — you could never
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# loosen one). Otherwise create it and assign the live process once.
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# CreateJobObject on an existing NAME returns a handle to that same job and
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# sets ERROR_ALREADY_EXISTS -- it does not make a second one. That is a more
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# reliable reuse probe than OpenJobObject, which needs the name to still be
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# resolvable in this session's namespace and quietly fails when it is not.
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# Getting this wrong would nest a second job, and nested CPU caps only ever
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# take the MINIMUM, so a loosened dial would silently do nothing.
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$ERROR_ALREADY_EXISTS = 183
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$job = [JobCpu]::CreateJobObjectW([IntPtr]::Zero, $jobName)
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$lastErr = [Runtime.InteropServices.Marshal]::GetLastWin32Error()
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if ($job -eq [IntPtr]::Zero) { throw "CreateJobObject failed: $([ComponentModel.Win32Exception]::new($lastErr).Message)" }
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$fresh = ($lastErr -ne $ERROR_ALREADY_EXISTS)
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if ($fresh) {
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$hProc = [JobCpu]::OpenProcess([JobCpu]::PROC_ACCESS, $false, $ProcessId)
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if ($hProc -eq [IntPtr]::Zero) { throw "OpenProcess failed: $([ComponentModel.Win32Exception]::new([Runtime.InteropServices.Marshal]::GetLastWin32Error()).Message)" }
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if (-not [JobCpu]::AssignProcessToJobObject($job, $hProc)) {
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$e = [Runtime.InteropServices.Marshal]::GetLastWin32Error()
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[JobCpu]::CloseHandle($hProc) | Out-Null
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throw "AssignProcessToJobObject failed ($e): $([ComponentModel.Win32Exception]::new($e).Message)"
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}
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[JobCpu]::CloseHandle($hProc) | Out-Null
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Write-Host "job : created '$jobName' and assigned the live process"
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} else {
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Write-Host "job : reusing '$jobName' (re-dialing, process untouched)"
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}
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$info = New-Object JobCpu+CPU_RATE
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if ($Off) {
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# An all-zero control block is rejected (ERROR_INVALID_PARAMETER): the API
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# validates the union against the flags. Releasing a cap therefore means
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# setting it to the full machine (100% == CpuRate 10000), not clearing it.
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$info.ControlFlags = [JobCpu]::ENABLE -bor [JobCpu]::HARD_CAP
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$info.RateOrWeight = [uint32]10000
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if (-not [JobCpu]::SetInformationJobObject($job, [JobCpu]::InfoClass, [ref]$info, 8)) {
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throw "SetInformationJobObject(off) failed: $([ComponentModel.Win32Exception]::new([Runtime.InteropServices.Marshal]::GetLastWin32Error()).Message)"
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}
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$proc.ProcessorAffinity = [IntPtr]([int64]([math]::Pow(2, $nCpu) - 1))
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Write-Host "cap : REMOVED (affinity restored to all $nCpu processors)"
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} elseif ($CoresWorth -gt 0) {
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# CpuRate is in 1/100 of a percent of TOTAL machine capacity, so a cores'
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# worth figure has to be divided by the processor count. Clamp to the API's
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# valid 1..10000 range; anything below 1 would silently mean "no limit".
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$rate = [int][math]::Round($CoresWorth / $nCpu * 10000)
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if ($rate -lt 1) { $rate = 1 }
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if ($rate -gt 10000) { $rate = 10000 }
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$info.ControlFlags = [JobCpu]::ENABLE -bor [JobCpu]::HARD_CAP
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$info.RateOrWeight = [uint32]$rate
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if (-not [JobCpu]::SetInformationJobObject($job, [JobCpu]::InfoClass, [ref]$info, 8)) {
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throw "SetInformationJobObject failed: $([ComponentModel.Win32Exception]::new([Runtime.InteropServices.Marshal]::GetLastWin32Error()).Message)"
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}
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Write-Host ("cap : {0:N2} cores' worth = {1:N2}% of machine (CpuRate {2})" -f $CoresWorth, ($rate/100.0), $rate)
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}
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if ($Cores -gt 0) {
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if ($Cores -gt $nCpu) { $Cores = $nCpu }
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$mask = [int64]([math]::Pow(2, $Cores) - 1)
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$proc.ProcessorAffinity = [IntPtr]$mask
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Write-Host "affinity: $Cores of $nCpu logical processors (mask 0x$('{0:X}' -f $mask))"
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}
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[JobCpu]::CloseHandle($job) | Out-Null
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$d = Measure-Draw $proc $SampleSeconds
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Write-Host ("result : {0:N2} cores' worth ({1:N2}% of machine)" -f $d.CoresWorth, $d.SystemPct)
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