IO Threading Model Used by SNA Server Services

ID: Q249068

The information in this article applies to:
  • Microsoft SNA Server, versions 3.0 SP1, 3.0 SP2, 3.0 SP3, 3.0 SP4, 4.0, 4.0 SP1, 4.0 SP2, 4.0 SP3
IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.

SUMMARY

To improve SNA Server performance and scalability, SNA Server services have been redesigned to use IO Completion Ports (IOCPs). The implementation of IOCPs began with the release of SNA Server 3.0 SP1; however, not all of the SNA Server services were updated at the same time. Services updated after SNA Server 3.0 SP1 are noted in the "More Information" section of this article.


MORE INFORMATION

The IOCP algorithms used by the various SNA Server services base the number of IO threads created on the number of processors in the SNA Server system.

The following details the algorithms used by specific SNA Server services to determine the number of IO threads each will create:

SNA Server service: (4 + 1) * (Number of Processors)

SnaBase service: (4 + 1) * (Number of Processors)

TN3270 service: 5 * (Number of Processors)

Note: The TN3270 service was updated to use IOCPs starting in SNA Server 4.0 SP2.

SnaPrint service:

5250 (AS/400) Printing: 5 * (Number of Processors)

Note: 5250 printing was updated to use IOCPs starting in SNA Server 4.0 SP3.

3270 Printing: 3270 printing does not make use of IOCPs. The SnaPrint service can create a maximum of 32 threads to service the 3270 print sessions. Each thread can support up to 32 sessions. Thirty-two threads supporting a maximum of 32 sessions each yields 1,024 sessions. Each of the first 32 3270 print sessions cause the Snaprint service to create a new thread. Therefore, if the Snaprint service is configured with 32 3270 print sessions, each print session is assigned to its own thread. Additional 3270 print sessions are assigned to the existing threads in a round-robin order to ensure that the sessions are balanced among the 32 threads. Please refer to the following article for more information on the thread/session usage for 3270 printing:

Q239861 SNA Server Host Print Service Supports 1,024 3270 Print Sessions
The rest of the SNA Server services (link services, TN5250, etc.) use the same IOCP algorithm that the SNA Client for Windows NT uses, which is the following:
2 * (Number of Processors)

Although the default number of IO threads should be sufficient in the vast majority of cases, it is possible to change the number of IO threads used by the SNA Server service and the TN3270 service. The following registry entries are used to set the number of IO Threads for the SNA Server service and the TN3270 service:

Note: Each thread created under Windows NT consumes approximately 1 MB of memory, so the number of IO threads should not be increased without reason. Adding a large number of additional threads has the potential of causing performance problems due to the additional memory being used.

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys and Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).


SNA Server Service

  1. Start Registry Editor (Regedt32.exe).


  2. Locate the following key in the registry:
    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SnaServr\Parameters


  3. On the Edit menu, click Add Value, and then add the following registry value:
    Value Name: NumberOfIOThreads
    Data Type: REG_DWORD
    Value: <Number of IO Threads>


  4. Quit Registry Editor.


If this registry entry is specified, the formula to determine the number of IO threads is as followis:
SNA Server: (NumberOfIOThreads + 1) * (Number of Processors)
If the registry entry is not present, the default value for NumberOfIOThreads for the SNA Server service is 4. The maximum value for NumberOfIOThreads is 64. If a value greater than 64 is specified, a value of 64 will be used.

An SNA Server computer that handles a large number of client log-on requests concurrently may experience some performance delays while processing the log-on requests. Increasing the number of IO threads in this scenario may help alleviate the delays.

A change was made in the way that the SNA Server DMOD handled log-on requests in SNA Server 3.0 SP2 because of the problem described in the following article:

Q168717 SNA Server Performance Degrades If Domain Controller Down
Because of this change, there can only be (NumberOfIOThreads/2 + 1) concurrent log-on requests in progress at one time. For a single processor system, this means that there can be three concurrent log-on requests in progress at one time. This guarantees that there are always enough IO threads to handle non-logon messages. Otherwise, all active SNA clients could stop responding if all of the IO threads were in use processing log-on requests.

If the number of log-on requests exceed the above limit, new log-on requests are queued internally in the DMOD until one of the pending log-on calls completes.

If NumberOfIOThreads is set to 10 in the registry on a single processor system, the DMOD can handle six concurrent log-on requests.

TN3270 Service

  1. Start Registry Editor (Regedt32.exe).


  2. Locate the following key in the registry:
    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\TN3270\Parameters


  3. On the Edit menu, click Add Value, and then add the following registry value:
    Value Name: NumIOThreads
    Data Type: REG_DWORD
    Value: <Number of IO Threads>


  4. Quit Registry Editor.


If this registry entry does not exist, the default value is 5.

Additional query words:

Keywords : sna3sp1 sna3sp2 sna3sp3 sna3sp4 sna4 sna4sp1 sna4sp2 sna4sp3
Version : WINDOWS:3.0 SP1,3.0 SP2,3.0 SP3,3.0 SP4,4.0,4.0 SP1,4.0 SP2,4.0 SP3
Platform : WINDOWS
Issue type : kbinfo


Last Reviewed: December 30, 1999
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