Sunday, June 28, 2009

Processor affinity on Windows server 2003/2008

This post describes a few methods of setting the processor affinity mask for a service, process or executable on a Windows Server 2003/2008 machine. There are several reasons to do this; increasingly as software becomes licensed per socket or even per core, but also separating or tuning the performance of applications on multi-core and hyper-threaded processors, i.e. to force usage of shared cache.

Of course this can be done through the task manager GUI (taskmgr.exe), but following the theme of command-line automation and scripting, I’ve found a few other methods that may be more efficient. The options below have different advantages/disadvantages and really depend on what you’re trying to achieve and how the process gets started.

In no particular order:


Use Windows System Resource Manager, which among other things can be used for persistent application to processor mappings. This requires 2003 enterprise, 2003 datacenter or server 2008. WSRM is useful when trying to run multiple applications on a single physical box, allowing granular control through a GUI (the configuration of which is persistent across reboots). WSRM also has a command-line interface, and can be used for accounting and to set per-processor utilisation, not just processor affinity.

Note that when I was testing WSRM, it detected calc.exe and matched it against a rule to limit 50% CPU usage, but using factorial of a large number still resulted in 100% CPU. WSRM uses soft caps apparently, only constraining usage when there is contention (I didn’t do any more testing to confirm). There is also a note about WSRM only supporting management of non-OS processes (I’m unsure of whether it would detect calc.exe as such).


Use PowerShell to set the processor affinity for one or more running processes. There’s an example script below, setting the processor mask of calc.exe to the first 4 processors. I like this method because the script is simple, it would be easy to schedule, works on x86 and x64, supports multiple processes of the same name and at least partly because it highlights just how easy administration with PowerShell is.

Note that if you use factorial of a large number with calc.exe (n!) you’ll generate100% CPU which can be useful for testing. The mask below is 0xf = 1111 – a mask allowing use of only the first four processors:

$calcSet = Get-Process -ProcessName "calc"
foreach ($calc in $calcSet) {$calc.ProcessorAffinity=0xF}

Start or psexec

Use a wrapper to launch the process, including either ‘start /affinity’ or ‘psexec -a’ to launch the application, setting affinity as required. Note that ‘start /affinity’ is available on 2003 server, Vista and 2008 server, but not XP.

psexec /a 0,1 calc.exe
start /affinity 1 calc.exe

If you’re trying to control a service, you could use instsrv/srvany to create a service that wraps the start or psexec command around the real service binary. For example, the commands below create another version of the spooler service that will only run on the first processor.

instsrv Test c:\util\srvany.exe
reg add hklm\system\currentcontrolset\services\test\Parameters
reg add hklm\system\currentcontrolset\services\test\Parameters /v Application /t reg_sz /d cmd.exe
reg add hklm\system\currentcontrolset\services\test\Parameters /v AppParameters /t reg_sz /d "/c start /affinity 1 C:\WINDOWS\system32\spoolsv.exe"


Use imagecfg.exe - a Windows Server 2000 Resource Kit Supplement utility - to modify the binary to set the processor affinity. This actually modifies the .exe – changing the initialisation section of the executable, and will ensure the affinity whenever the executable is run. Note that in my testing, modifying an x64 binary with this utility did not set the affinity correctly, only x86 binaries were successful (running on x64 or x86 OS). If you’re using calc.exe to test this – use a copy in another folder, otherwise windows file protection will replace the updated binary, undoing any change you make.

The mask below is 0xf = 1111 – a mask allowing use of only the first four processors:

imagecfg -a 0x0F c:\temp\calc.exe


Use procaff.exe (a third-party utility) to set the processor affinity of a running process. This could be run as startup scheduled task, setting the mask of any process started as a service (you might need to build in some logic to wait for the processes to start). Note that this utility worked on both x86 and x64 systems while testing, but it’s limited in that if there are multiple processes with the same name, it requires a PID. This is easy enough to script using a ‘for’ loop and pslist to identify the PIDs and then call procaff.

procaff /set 1 calc.exe

Boot.ini and HAL options

Some rather more severe options – reducing the OS to only one CPU:

  1. The /NUMPROC switch in boot.ini on the server. This will set the number of processes Windows will see when booted. I assume this will just apply a mask at startup, but in a multi-CPU multi-core server I haven't tested to confirm the cores are paired or whether the first core from each CPU is presented.
  2. The /ONECPU switch in boot.ini on the server. This will tell Windows to use only one CPU, limiting any application running on the OS. On a multi-core server, the OS would use only one core of one package. I successfully tested this on 2003 enterprise OS.
  3. Downgrade the OS from a multiprocessor HAL to a uniprocessor HAL, removing support for multiple CPUs from the OS. I didn’t actually test this method, but it’s something I used to do on NT4, and I don’t see why it wouldn’t work on 2003/2008.

Plug-and-Play device interrupts

Use Intfilter.exe - allowing you to perform similar functionality to drivers – binding interrupts to a particular processor, useful to control plug-and-play driver-level CPU binding.



Boot INI Options Reference

HAL options after Windows XP or Windows Server 2003 Setup

Available Switch Options for Windows NT Boot.ini File

How to Manually Add Support for a Second Processor

Windows System Resource Manager: Frequently Asked Questions

Windows System Resource Manager

Best Practices for Managing Applications with Process Control

The Context Switch Action (xperf)

Monitoring Context Switches and Threads

Analyzing Processor Activity

Monitoring Activity on Multiprocessor Systems

Windows Performance Toolkit x86 and x64 v4.1.1

Windows Performance Toolkit

Intfilter to manage drivers:;en-us;252867

Wayne's World of IT (WWoIT), Copyright 2009 Wayne Martin. 


Ryan Fowler said...

Thanks for the post. It's quite helpful.

Mike O said...

I have seen conflicting information on the imagecfg mask. Do you have some additional detail on the mask and the input it accepts, as well as how that input affects which CPU(s) is used? Thanks.

Wayne Martin said...

I think it's a binary bit mask, the examples below might help:
- 00000001, 0x1, use only the first processor
- 00000011, 0x2, use the first two
- 00000111, 0x7, use the first three (not sure if this is valid)
- 00001111, 0xf, use the first four
- 11001100, 0xCC, In a quad-CPU dual-core server, use only the second and fourth CPU packages (and two cores within each package)
- 0000111100001111, 0xF0F, In a quad-cpu quad-core server, use only the first and third CPU packages (and four cores within each package)

I think setting the affinity on x86 systems is limited to 32 processors (two dwords), you need 64-bit to set the affinity on 33-64 processors.

Paul Mulroney said...

I think the example you gave is incorrect. 00000011 is actually 0x3, not 0x2.

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Sakib Raj said... "Windows Server 2008 provides a solid foundation for all of your server workload and application requirements while being easy to deploy and manage."
Windows Server 2008 Support


After starting the wrapper service that calls a command shell and launches the actual service exe, the command shell does not exit after the service executable exits...

Did anyone face this issue?

Tom Watson said...

I had issues with setting a service affinity using the Srvany.exe wrapper in the article as well.

What I did instead was to get the ProcAff tool from and scheduling it to change the service's executable 10 minutes after system startup. What is essesntial however, is that the scheduled task must run using local SYSTEM account.

My example was a 64 bit machine, so my scheduled commandline was :-

C:\ProcAff\x64release\procaffx64.exe /set 1 SomeProcess.exe

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About Me

I’ve worked in IT for over 13 years, and I know just about enough to realise that I don’t know very much.