Microsoft KB Archive/146004

= SNMP Counter Values Are Unreliable in SMP Computers =

Article ID: 146004

Article Last Modified on 2/21/2007

-

APPLIES TO


 * Microsoft Windows 2000 Server
 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows 2000 Professional Edition
 * Microsoft Windows 2000 Datacenter Server
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition

-



This article was previously published under Q146004





SYMPTOMS
Windows NT simple network management protocol (SNMP) counters, such as output queue length, may yield unlikely results in symmetric multiprocessor (SMP) computers. The SNMP counters can be viewed using Snmputil.exe from the Windows NT Resource Kit, or Performance Monitor using the Network Interface object. For example:

snmputil getnext public 2.2.1.21.1

Variable = interfaces.ifTable.ifEntry.ifOutQLen.2

Value = Gauge - 3749

NOTE: In normal operation, the output queue length counter should rarely rise above zero, when it does, it should return to zero very quickly.



CAUSE
This problem occurs because Windows NT TCP/IP does not use interlocked operations to maintain its statistics counters. The SNMP counters are maintained in the TCP/IP code.

In the case of output queue length, the counter is incremented every time a packet is indicated to NdisSend, and decremented when NDIS completes the send successfully by returning a status of success, or returning status pending and later calling ARPSendComplete.

Because spinlocks are not used in this code to guarantee the integrity of the counters, SMP computers can affect the counter values during retrieval, causing unlikely values to be reported.

The decision to not implement spinlocks to guarantee the integrity of the SNMP counters is based on performance concerns and will not be changed.



STATUS
Spinlocks were left out of the TCP/IP code for counter operations intentionally for performance reasons. This will not be fixed.

Additional query words: perfmon

Keywords: kbnetwork kbsnmp KB146004

-

[mailto:TECHNET@MICROSOFT.COM Send feedback to Microsoft]

© Microsoft Corporation. All rights reserved.