Improving Enumeration Performance

Enumerations tend to use a significant amount of system resources. Therefore, you should try to optimize the WMI enumeration process if you plan on performing enumerations on a large group. Scripts can also use a query to avoid performance degradation in "For each….Next" operations with a large set. For more information, see Querying WMI.

The following procedure describes how to improve enumeration performance.

To improve enumeration performance

  1. Set the lFlags parameter to allow semisynchronous return of the data with an enumerator that discards each item from WMI as it is delivered. For more information, see Calling a Method.

    The following C++ code example shows how to use the WBEM_FLAG_RETURN_IMMEDIATE and WBEM_FLAG_FORWARD_ONLY flags.


    In VBScript or Visual Basic, use the scripting flags WbemFlagReturnImmediately and WbemFlagForwardOnly from WbemFlagEnum. The combined value of these flags is decimal 48.

    The scripting and parameter flags cause the following behavior:

    • The WBEM_FLAG_RETURN_IMMEDIATE or wbemFlagReturnImmediately flag requests semisynchronous behavior. The call to create the enumerator returns immediately. You can then begin to traverse the object set that you receive.
    • The WBEM_FLAG_FORWARD_ONLY flag or wbemFlagForwardOnly flag requests an enumerator that you cannot rewind. That is, WMI can release an object after you view the object.

    In situations where the enumeration is large and the application is very fast, using forward-only enumerators with semisynchronous processing allows WMI to hold on to far fewer objects, thereby increasing response time and memory performance significantly.

    The following VBScript code example shows how to make a call using the combined wbemFlagReturnImmediately and wbemFlagForwardOnly flags to obtain a collection of events from an event log.

    Set Events = GetObject("winmgmts:").ExecQuery _
         ("SELECT * FROM Win32_NTLogEvent " _
          & "WHERE Logfile = 'System'",,48)
  2. Whenever possible, avoid using CreateInstanceEnum in C++ or SWbemServices.InstancesOf, and instead use ExecQuery.

    The ExecQuery method queries WMI using database technologies, while CreateInstanceEnum or SWbemServices.InstancesOf enumerates WMI objects. Specifically, ExecQuery can request specific subsets of data that the enumerating methods cannot.

    Because some providers do not have querying capabilities, WMI provides a "post filter" feature that allows WMI to discard instances that do not fulfill a query's specifications. Whether a particular provider takes advantage of this feature is up to the provider author.

  3. Experiment with different queries to determine what gives you the best performance.

    For example, WMI seldom efficiently processes queries with WHERE clauses of the form Prop1 < "x". In contrast, WMI normally processes queries of the form KeyProp1 = "x" efficiently.

For more information, see Enumerating WMI.