Communication Server for AIX Tuning and Performance Tips


About this document
    Related documentation
Communications buffers (mbufs)
Setting maxmbuf
SNA DLC link parameters
LAN-based DLC characteristics
Paging space
File transfer applications

About this document

This document contains tips for network tuning in an SNA environment. It applies to AIX Versions 4.x.

Related documentation

For more information on network tuning in an SNA environment, refer to publications Communications Server for AIX Planning and Performance Guide and IBM eNetwork Communications Server for AIX: Understanding and Migrating to Version 5: Part 2 - Performance.

The product documentation library is also available:

Communications buffers (mbufs)

The mbuf pool is a shared resource that must be managed at the system level. Communications Server for AIX uses mbufs to send and receive data across the network, but it is just one of the subsystems that uses mbufs. Communications Server for AIX mbuf resource utilization can affect the performance of other subsystems such as TCP/IP, NFS, and AFS.

There are two ways to determine how much memory should be be made available as communications memory buffers (mbufs). This amount of memory is defined using the maximum number, or maxmbuf.

  1. "4096 X number of Comm Adapters"

    This method counts the total number of communications adapters in the system (whether being used for SNA or not), and then multiplies this number by 4096. The result is used as the setting for maxmbuf.

    For example, if there are two token ring adapters that are being used for SNA and two Ethernet adapters that are being used for TCP/IP, the calculation would be 4 adapters multiplied by 4096, or 16384. 16384 would be the number used for maxmbuf.

  2. "25% of RAM"

    SNA generally requires more memory for buffers than other applications. With the larger memory sizes that have become available, SNA will usually operate efficiently if the maxmbuf parameter is set to a number equal to 25% of real memory. In most circumstances, any number larger than 64MB (65536) will not yield any appreciable benefit.

    For example, if there is 128Mb of real memory on the machine, set the maxmbuf parameter to 32Mb (32768). If there is 512Mb of memory, set maxmbuf to the recommended maximum of 64Mb (65536).

    If SNA_SRF errors are being logged in the system error log, this parameter should be checked.

Setting maxmbuf

To check the current setting of this parameter, enter the following:
           lsattr -El sys0 |grep maxmbuf
To change this parameter, enter the following:
           chdev -l sys0 -a maxmbuf=new_value
For example, to set maxmbuf to its maximum value of 64MB, enter the following:
           chdev -l sys0 -a maxmbuf=65536
NOTE: On AIX 4.2 and AIX 4.3, the default for maxmbuf is 0. This value is known to cause problems with SNA. Please set maxmbuf to any value other than 0.

SNA DLC link parameters

Certain SNA DLC profile parameters significantly impact throughput of file transfer applications. For token ring and Ethernet, use the following guidelines.

For an SDLC connection, the Receive_Window_Count does not exist. With SDLC, the primary_repoll_time_out parm setting must allow enough time for the largest frame to travel to and from the secondary station. Otherwise, unnecessary polls will flood the link, eventually bringing it down.

If the link speed is X bits per second and the largest frame is Y bits, then the primary_repoll_time-out should be larger than 2(Y/X).

LAN-based DLC characteristics

The Token Ring, Ethernet, and FDDI Data Link Controls contain a parameter called the Depth of receive queue. This queue is logically between the device handler and the data link control and holds asynchronous event notifications such as the receipt of data packets. This queue defaults to a depth of 32 entries and can be changed by the operator if log entries indicate that the device handler is overrunning the DLC due to heavy system activity. This attribute should be changed in small increments because it directly affects system storage availability.

To change this value, use the following fastpath command.

           smit devices
Then follow these steps:

  1. Select Communication.

    Select the type of communication from the list displayed (for example, TOKEN RING ADAPTER).

  2. Select Services.

  3. Select Data Link Control.

  4. Select Change/Show Data Link Control.

  5. Change the parameter value.


A rough estimate of the amount of memory required to run a certain number of sessions is that one additional megabyte of memory will support about seven additional sessions. The exact number will depend on traffic.

Paging space

A paging space is a dedicated area of fixed disk storage used to temporarily store information from real memory. When activity increases and the amount of available free memory decreases, "pages" of programs or data that have not been referenced recently are moved to fixed disk to allow real memory to be used for immediate requirements.

For systems with less than 512Mb of real memory, the value recommended for paging space is TWO TIMES the amount of real memory. For systems with 512Mb of real memory or greater, the recommended value for paging space is an amount at least EQUAL to the amount of real memory.

To display the amount of memory, enter

           lsattr -El sys0 | grep realmem
To display total paging space and the approximate amount used, enter

           lsps -s
It is advisable to keep the paging space used below 75% to ensure that SNA resources will be available. If these resources must be "paged" out of real memory, it is important that enough space is available to prevent SNA from experiencing errors that could result in total failure of SNA communications. If the percent used is over 75%, additional paging space should be added.

File transfer applications

When using file transfer applications, the SNA LU 6.2 Mode Profile can be configured to maximize throughput. Use the following guidelines.

[ Doc Ref: 9064778089346     Publish Date: Jan. 03, 2001     4FAX Ref: 9777 ]