Using DOSKBD to Modify MS-DOS Keyboard Polling Detection

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Standard MS-DOS applications will use nearly 100 percent of the system CPU because they constantly look for keyboard input, whether the application is active (checking for keyboard input to be more responsive) or idle. Obviously, no application should be allowed to monopolize the CPU. In a Terminal Server environment, keyboard polling can make an MS-DOS application sluggish for a single user, or can seriously limit the number of simultaneous users.


There are two ways to modify an application's keyboard polling. The first is to modify the application's properties, and the second is to use the MS- DOS utility, DOSKBD.

Determining that a Problem Exists

Application performance problems will be obvious to the user, but to get a more precise picture of application resource use, use Performance Monitor. If you suspect an MS-DOS application is monopolizing the CPU, Performance Monitor can tell you precisely how much CPU time the application is using. To avoid false readings, make sure that Terminal Server is running as few system-processes as possible. Run only Performance Monitor and a single client connection from which to run the MS-DOS application.

In Performance Monitor, open a graph that displays total system CPU usage, every second. Note the CPU usage before you have the client run the MS-DOS application. This will give you a baseline.

Now have the client run the MS-DOS application and note the increased CPU usage.

Lowering CPU Usage through Application Properties

Have the client close the MS-DOS application. At the Terminal Server, run Windows NT Explorer, and highlight the application's executable file. Right- click this file and select Properties from the popup list. On the Properties Screen, select MISC (miscellaneous). You will find a slider bar called Idle Sensitivity. This setting tells the system how much CPU time to allow the application to use while it is idle. Low sensitivity allows the application to use more CPU time. High sensitivity reduces allowable CPU time. In this case, you want to increase sensitivity. Every application will be different, so determining the correct sensitivity level will necessitate some trial and error.

NOTE: If you are familiar with Citrix Winframe, you may have used the PIF Editor to modify an MS-DOS application's properties. Terminal Server uses the Windows NT 4.0 method: right-click the application name in Windows NT Explorer, and open the application's Properties. Whenever properties are changed, an application shortcut is created. You can place this shortcut on the user's desktop, folder, or START Menu. Launching the application from the shortcut activates the properties you select. However, running the application's executable from a command line will not activate the properties you select.

After you modify the application properties and exit the Properties screen, a shortcut will be created with a default name identical to the application executable name. Use this shortcut to start the application. You can modify the shortcut name if you want to create multiple instances with different properties.

Have the client run this shortcut while you monitor Performance Monitor for decreased CPU use.

NOTE: Modifying the application properties affects ONLY idle time. It does not decrease CPU use while the application is active (for example, when the user is typing).

The DOSKBD Utility

The DOSKBD utility can decrease an MS-DOS application's CPU usage from keyboard polling during active and idle times. It is also much more complex than simply modifying the application properties. What is also much more complex?

DOSKBD works by monitoring how frequently the MS-DOS application checks the keyboard buffer for input. If it checks too frequently during a specific interval (by default, a single system timer tick), the application is suspended, or put to sleep, for a short period of time.

The default values for DOSKBD are in effect whenever an MS-DOS application is started. The defaults, switches, and usage are detailed below. If you need to modify any of the default values, run DOSKBD with the new values from the same command prompt (or within the same MS-DOS session) that you use to run the application. The easiest way to accomplish this is to create a batch file that runs DOSKBD with the appropriate settings and then runs the application. An alternative to using a batch file is to create a custom autoexec.nt file and associate it with the MS-DOS application's shortcut properties.

By using batch files to run your MS-DOS applications, you can set different values for DOSKBD as needed.

These are the default values for DOSKBD. These are the values in effect, whenever an MS-DOS window is opened.

  • Number of milliseconds in a system timer tick is 10
  • DetectionInterval = 1 tick(s) (10 msec)
  • DetectProbationCount= 80
  • InProbationCount= 35
  • msAllowed= 0
  • msSleep= 100
  • BusymsAllowed= 60
  • msProbationTrial= 2500
  • msGoodProbationEnd= 2500
The poll detection algorithm is driven by how many keyboard polls happen within a polling detection interval. All units of time are rounded to a system timer tick. If too many keyboard polls occur within a detection interval, the application is put on probation and punished by being put to sleep for a predetermined amount of time. When the application is on probation, the algorithm can be set up to punish the application after a fewer number of polls.

Activities such as updating the screen, doing file I/O, or getting mouse or keyboard activity cause the system to mark the application as busy. When the application is determined to be busy, it is awakened if it is being punished. In addition, the application is taken off probation.

If an application polls DetectProbationCount number of times within a given timer tick, the application is put into probation if BusymsAllowed number of milliseconds have elapsed since the last time the application was detected as busy. When the application is put into probation for msAllowed milliseconds, it is punished for exceeding the allowed number of polls within a timer tick by being put to sleep for msSleep number of milliseconds.

While the application is on probation, the number of allowed polls is reduced to InProbationCount from DetectProbationCount. After exceeding the allowed number of polls, it is put to sleep as long as msAllowed milliseconds have elapsed since the application probation.

The application is taken off probation if msGoodProbationEnd milliseconds have elapsed since the application was last punished. In addition, every msProbationTrial milliseconds, the allowed number of polls in a timer tick is increased to DetectProbationCount. As long as the application is not punished, the allowed number of polls stays at the higher level. If the application exceeds that level, the allowed level of polls is reduced to InProbationCount.

DOSKBD Command-Line Switches, Syntax, and Values

At a command prompt, type DOSKBD /? to display how to use DOSKBD. To display the current settings, type DOSKBD. The number of milliseconds in a system timer tick is also displayed.








   DOSKBD displays the current settings.

   DOSKBD /DEFAULTS resets all tuning parameters to the system defaults.

   Any DOSKBD command line ending with /Q will not display any information.

   NOTE: Valid range for all values (represented by nnn) is 0 to 32767.


   The number of peeks in the detection interval required to force the
   application into the probation state and to sleep the application.


   The number of peeks in the detection interval required to sleep the
   application when the application is in probation. Should be <=


   The number of milliseconds the application is allowed to be in the
   probation state before the system starts sleeping the application.


   The number of milliseconds that the application is put to sleep.


   When the application is detected as 'busy', the application cannot be
   put into the probation state for this # of milliseconds.


   When the application is in probation, DETECTPROBATIONCOUNT is used
   instead of INPROBATIONCOUNT every MSPROBATIONTRIAL milliseconds.


   When the application is in probation it must avoid being put to sleep
   for this # of milliseconds in order to be removed from probation.


   The length of time (in ticks) used to count up the number of polling

   /STARTMONITOR [appname]

   Start gathering polling statistics.


   Stop gathering polling information and display statistics.

How to Use DOSKBD

The typical scenario (typical scenario for what?) is that an MS-DOS application runs fine for a single user, but the system slows down when more users start using the application. When the system slows in this way, it is possible that the polling detection is not being aggressive enough to put the application into probation. Observe how much CPU the application is using while doing nothing, and while doing common operations. Then exit the application and use DOSKBD to refine the polling parameters. Remember that DOSKBD settings affect only the MS-DOS session in which they are set, so it is advisable that you modify your DOSKBD settings and run the MS-DOS application within a batch file, or through the use of a custom autoexec.nt file.

Changing parameters by about 30 percent per try is recommended.

To more aggressively take the CPU away from the application, first try decreasing DetectProbationCount and InProbationCount.

If that does not help or does not help enough, try lowering BusymsAllowed by 10 milliseconds per try. The default of msAllowed is already 0 (zero). If polling is being detected, the CPU should be significantly less than 100 percent by now.

At this point it is important to make sure that the application is still Able to respond in all the ways that it will be used. If it is not responsive, you have gone too far, and you must back off some of the settings. To further reduce the CPU used, msSleep can be increased. Use caution while increasing msSleep, because some applications become unresponsive or jerky if this value is increased too much.

As you reduce the amount of CPU that the application is using, while ensuring that the application is still responsive, the system should be able to support a greater number of concurrent users.

Some applications may use close to 100 percent CPU, regardless of how aggressive you make the polling detection. Keyboard polling is a common problem, but applications may monopolize the CPU in other ways.

If an application is not getting enough CPU, the polling detection is probably too aggressive, or the application needs more time to run before it is punished. You can increase DetectProbationCount and InProbationCount to lengthen the time it takes to detect that the application is polling. You can increase msAllowed from zero to give the application additional execution time, before it is punished for being on probation. You can also give the application more CPU while it is being punished by reducing the value of msSleep.

All these possibilities increase the amount of CPU that the application gets, which reduces the number of users who can simultaneously do useful work on the system.

NOTE: DOSKBD is used only for MS-DOS applications. However, Windows applications may also monopolize the CPU by checking the message queue too frequently. This behavior can be modified through the system registry. See the article Performance Tuning CPU Usage for 16-bit and 32-bit Windows Applications for more information.


Article ID: 186560 - Last Review: November 1, 2006 - Revision: 2.1
  • Microsoft Windows NT Server 4.0, Terminal Server Edition
kbinfo KB186560

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