No Load Balancing When You Invoke Many APPC Conversations

• Microsoft SNA Server for Windows NT, versions 2.0, 2.1, 2.11 and 2.11 Service Pack 1

SYMPTOMS

LU6.2 load balancing problems have been observed under two conditions:

1) When a single program allocates multiple conversations:

If an APPC or CPIC program allocates multiple conversations at the same time on an SNA Server client or server and SNA Server is configured to support LU6.2 load balancing, the conversations may not load evenly across more than one SNA Server.

For example, if a program tries to allocate 16 concurrent conversations across two SNA Servers, which are configured to support 10 sessions each, the program may allocate 10 conversations through the first server and two conversations through the second server. The remaining four conversations are queued against the first server.

2) When multiple APPC or CPIC programs allocate conversations at once:

For example, if multiple programs are started and each tries to allocate one conversation, where two SNA Servers are available, sessions tend to be allocated from only one SNA Server.

CAUSE

This problem occurs when the LU6.2 sessions are not activated before the application attempts to allocate the conversations.

When an APPC or CPIC application allocates a conversation, the SNA Server APPC interface attempts to locate an appropriate, available LU6.2 session on any SNA Server running in the domain or subdomain. It selects a server that has an active LU6.2 session before it tries to select a server that has an inactive LU6.2 session.

If multiple conversations are allocated at the same time by a program, the APPC interface does not randomly select between servers if they all start LU6.2 sessions. This can cause multiple conversation startup requests to be queued against a single SNA Server rather than load balancing across multiple SNA Servers.

RESOLUTION

To work around this problem:

1. Configure the SNA Server and the APPC or CPIC program to support LU6.2 load balancing. Ensure that the configuration is correct. For additional information, please see the following article in the Microsoft Knowledge Base:

         ARTICLE-ID: Q128244
         TITLE     : SNA Server Load Balancing and Hot Backup
   
   

2. Start the LU6.2 session before the application starts. LU6.2 sessions can start automatically by setting the "autoactivation limit" in the SNA Server APPC Mode entry (on SNA Server Admin) to the number of sessions.

   If automatic LU6.2 session startup is not acceptable, an update to the SNA Server APPC interface is available to improve load balancing (WAPPC32.DLL for Windows NT and Windows 95 clients, and WINAPPC.DLL for Windows 3.x clients).

3. Configure extra LU6.2 sessions on each SNA Server so that the SNA Server APPC random server selection locates a server with an available LU6.2 session.

   For example, if an application allocates 16 concurrent conversations against two SNA Servers, each SNA Server should support at least 10 sessions each. If you configure each server to support only eight sessions, some conversations may still be queued against a server that has all LU6.2 sessions in use.

STATUS

Microsoft has confirmed this to be a problem in SNA Server 2.11 and SNA Server 2.11 Service Pack 1. This problem was corrected in the latest Microsoft SNA Server 2.11 U.S. Service Pack. For information on obtaining the service pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K