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Q156967: SNA Server Performance Degrades Running LUA Application

Article: Q156967
Product(s): Microsoft SNA Server
Version(s): WINDOWS:2.0,2.1,2.11,2.11 SP1,3.0
Operating System(s): 
Keyword(s): kb3rdparty kbprogramming
Last Modified: 13-JUN-2001

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The information in this article applies to:

- Microsoft SNA Server, versions 2.0, 2.1, 2.11, 2.11 SP1, 3.0, on platform(s):
   - the operating system: Microsoft Windows NT 
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SYMPTOMS
========

When you run an LUA application (or 3270 application that uses the LUA flavor of
the SNA Server 3270 Emulator Interface Specification), SNA Server performance
may degrade if the application is receiving a large number of frames and SNA
exception response protocol is being used on the session.

CAUSE
=====

This problem can occur if exception response protocol is used on an LU type 0,
1, 2, or 3 session, and the application does not periodically send "courtesy
acknowledgments" to SNA Server, as documented in the SNA Considerations section
in Chapter 2 of the SNA Server "LUA Programmer's Guide" (included below).

If an LUA application developer encounters this problem, the application should
issue "courtesy acknowledgments" periodically (say, after every 10 frames
received or so), as described below.

If a customer suspects a problem in this area, see the information below for
instructions on generating SNA Server traces to provide to the LUA application
developer or to SNA Server support personnel.

MORE INFORMATION
================

Chapter 2 of the SNA Server "LUA Programmer's Guide" includes the following
information regarding "courtesy acknowledgments":

"SNA Server keeps a record of requests received from the host in order to
correlate any response sent by the application with the appropriate request.
When the application sends a response, SNA Server correlates the response with
the data from the original request, and can then free the storage associated
with it.

If the host specifies exception response only (a negative response may be sent
but a positive response should not be sent), SNA Server must still keep a record
of the request in case the application subsequently sends a negative response.
If the application does not send a response, the storage associated with this
request cannot be freed.

Because of this, SNA Server allows the LUA application to issue a positive
response to an exception-response-only request from the host (this is known as a
courtesy acknowledgment). The response is not sent to the host, but is used by
LUA to clear the storage associated with the request."


If the LUA application does not issue courtesy acknowledgments in this scenario,
the following behavior may be observed using Microsoft Windows NT Performance
Monitor utility:

- The "private bytes" memory counter for the SNASERVR.EXE process will steadily
  increase while the LUA application receives data.

- Processor CPU usage may steadily increase

- Data throughput on the LUA session will decrease

Also, if SNA Server tracing is enabled, the SNA Server may encounter unexpected
delays up to 45 seconds before responding to client requests.

To determine if this problem is occurring, the following traces should be
captured and provided to the LUA application developer, or to SNA Server support
personnel:

- Stop the LUA application

- Run SNATRACE, focus on the SNA Server service and turn on Data Link Control
  (DLC) and 3270 message tracing, set the trace file size to a large value
  (say, 5000000 or 5 MB) and select Apply.

- Start the LUA application and run a sample transaction.

- Return to SNATRACE and select Clear All. The resulting trace files can be
  found in <Snaroot>\System\Nodemsg?.trc.

To send a courtesy acknowledgment, the LUA application should write a response
containing a zero-length data message, setting the RH to indicate a response and
by setting the TH to have the sequence number (SNF) of the RECEIVE verb that is
being acknowledged. A SLI_SEND or RUI_WRITE indicates a response if it has the
response bit set in the RH in the verb control block. The bits to set are:

  lua_rh.rri = 1 /* response */
  lua_rh.ri = 0 /* positive response*/

Also, if you use an SLI application, the lua_message_type in the SLI_SEND command
must be set to indicate a response (LUA_MESSAGE_TYPE_RSP). This is described in
Chapter 4 of the [ASCII 147]LUA Programmer's Reference.[ASCII 148]

Additional query words: sp1 prodsna

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Keywords          : kb3rdparty kbprogramming 
Technology        : kbAudDeveloper kbSNAServSearch
Version           : WINDOWS:2.0,2.1,2.11,2.11 SP1,3.0
Issue type        : kbprb

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