Performance Management Guide

Tuning Asynchronous Connections for High-Speed Transfers

Async ports permit the connection to a computer of optional devices such as terminals, printers, fax machines, and modems. Async ports are provided by adapter devices such as the 8-, 16-, or 64-port IBM adapters or the 128-port Digiboard adapter, which reside on the Micro Channel and provide multiple asynchronous connections, typically RS232 or RS422. Many adapters, such as the three IBM async adapters mentioned above, were originally designed for servicing terminals and printers, and so are optimized for output (sends). Input processing (receives) is not as well optimized, perhaps because the assumption was once made that users could not type very fast. This is not a great concern when data transmission is slow and irregular, as with keyboard input. It becomes a problem with raw-mode applications, where massive chunks of input are transmitted by other computers and by devices such as fax machines. In raw-mode processing, data is treated as a continuous stream; input bytes are not assembled into lines, and erase and kill processing are disabled.

Async Port Tuning Techniques

While some adapters have inherent limitations, the guidelines and techniques described here can provide better performance from these adapters for raw-mode transfers.

A shell script containing appropriate stty commands to implement most of the following tuning techniques is given at the end of the section.

For input processing, using the echo option is expensive, as it increases the time per character. Character echo is useful for canonical user input but is probably not necessary for most raw-mode applications. Example:
```
# stty -echo < /dev/ttyn
```
Increase the value of the vmin variable for each TTY from the default of 4. The vmin variable value is the minimum number of bytes that should be received when the read is successful. The value chosen for the vmin variable should be the lesser of the application data-block size or 255 (the maximum allowed). If the application block size is variable or unknown, vmin should be set to 255. Setting the vmin variable to 255 will result in fewer read executions and will reduce CPU utilization by 15 to 20 percent for file-transfer programs.
Except on the 128-port adapter, set vtime > 0 to prevent an indefinite block on read. If the vtime variable is set to zero on the 128-port adapter, POSIX line-discipline processing will be offloaded to the adapter hardware, significantly reducing CPU processing.
For raw-mode sends where output translations are not needed, turn off the opost option in the POSIX line discipline. This will help the CPU performance by reducing the output path length. For file-transfer applications, which move large amounts of data on TTY lines, this can reduce CPU utilization by 3 times. Example:
```
# stty -opost < /dev/ttyn
```
Because the 64-port adapter is prone to unpredictable data overruns at higher baud rates when XON/XOFF is used for pacing, avoid the risk of losing data by using RTS/CTS hardware pacing instead.
Because the 64-port-adapter concentrator boxes have a limited bandwidth and saturate at higher baud rates, adding more ports to a saturated concentrator will decrease the performance of all ports connected. Instead, add another concentrator and continue until it is saturated or you have run out of CPU.

Also, concentrator saturation is a concern because, if the concentrator box approaches overload, no additional throughput is accepted. The effective baud rate is lowered, and there is a noticeable slowdown in work.

Shell Script fastport.sh for Fast File Transfers

The fastport.sh script is intended to condition a TTY port for fast file transfers in raw mode; for example, when a fax machine is to be connected. Using the script may improve CPU performance by a factor of 3 at 38,400 baud. The fastport.sh script is not intended for the canonical processing that is used when interacting with a user at an async terminal, because canonical processing cannot be easily buffered. The bandwidth of the canonical read is too small for the fast-port settings to make a perceptible difference.

Any TTY port can be configured as a fast port. The improved performance is the result of reducing the number of interrupts to the CPU during the read cycle on a given TTY line. To configure a TTY port as a fast port, do the following:

Create a TTY for the port using the command smitty -> Devices -> TTY -> Add a TTY), with Enable LOGIN=disable and BAUD rate=38,400.

Create the Korn shell script named fastport.sh, as follows:

     #****************************************************************
     #
     #            Configures a fastport for "raw" async I/O.
     #
     #****************************************************************
     set -x
     sync;sync
     i=$1
     if [ $i -le 100 ]
     then
     # for the native async ports and the 8-, 16-, and 64-port adapters
     # set vmin=255 and vtime=0.5 secs with the following stty
      stty -g </dev/tty$i |awk ' BEGIN { FS=":";OFS=":" }
       { $5="ff";$6=5;print $0 } ' >foo
     # for a 128-port adapter, remove the preceding stty, then
     # uncomment and use the
     # following stty instead to
     # set vmin=255 and vtime=0 to offload line discipline processing
     # stty -g </dev/tty$i |awk ' BEGIN { FS=":";OFS=":" }
     #  { $5="ff";$6=0;print $0 } ' >foo
      stty `cat foo ` </dev/tty$i
      sleep 2
     # set raw mode with minimal input and output processing
      stty -opost -icanon -isig -icrnl -echo -onlcr</dev/tty$i
        rm foo
      sync;sync
     else
        echo "Usage is fastport.sh < TTY number >"
     fi

Run the script for TTY number with the following command:
# fastport.sh ttynumber