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You can use the sar -y and sar -g commands (or their mpsar equivalents for SMP) to examine the performance of the terminal (tty) and serial I/O drivers.
sar -y reports character processing and hardware interrupt activity in the tty and serial drivers:
23:59:44 rawch/s canch/s outch/s rcvin/s xmtin/s mdmin/s 23:59:49 10 10 25 11 11 0 23:59:54 12 10 34 14 14 0 23:59:59 8 8 12 8 8 0For dumb serial cards using 8250 or 16450 UARTs, the ratio of the number of interrupts received from the serial card per second,Average 10 9 24 11 11 0
rcvin/s, to the number of raw characters received per
second, rawch/s, should be close to 1.
Non-intelligent (dumb) serial cards that use
the 16550 UART can buffer up to
14 characters before generating an interrupt. In this case, the
ratio could be as low as 0.07 but will usually be nearer 1 if
continuous input is not arriving.
Intelligent serial cards move many more characters per interrupt and
so cause much less loading of the CPU.
If the users of your system are logged in over serial lines,
you should monitor rawch/s and
rcvin/s over time to gain an
impression of typical values for the rates of
raw character input and interrupt activity for the serial card(s)
on your system. If you are using intelligent multiport cards,
these are usually supplied with software that you can use to
diagnose their performance.
You should also examine the values of rawch/s and
rcvin/s if you
suspect that a bad line or chattering terminal is
generating spurious interrupts. If any serial devices are being
used with modem control, you can also examine the number of
modem interrupts per second shown by mdmin/s.
These interrupts are generated when the modem-control lines, such as
RTS, and CTS, change state.
The CPU has to handle all interrupts that it receives from serial cards. If the number it has to handle is very high, it will be interrupt bound and may be unable to allocate sufficient time to running user applications. In extreme cases, characters may be lost if interrupts arrive while other interrupts are still waiting to be processed. This is known as interrupt overrun.
When using fast modems, too many characters may arrive in the UART's buffer while the interrupt handler is trying to process them. This is referred to as high interrupt latency. In such a case, the buffer may lose the last character to arrive. To prevent this, you need to decrease the number of characters in the buffer that will cause the UART to generate an interrupt (the receive interrupt trigger level). You should also increase the values of the kernel parameters NCLIST and TTHOG to match the increased demand on the terminal and serial drivers. TTHOG controls how many characters are allowed to be on the raw queue of the terminal driver (see ``Processing of input and output characters by the character list-based terminal driver'') before the driver will automatically dispose of them. You must also increase the number of character list blocks, defined by NCLIST, to allow the raw queue to grow to such a size. See ``Configuring your system for high-speed modems'' for more details.
sar -g reports interrupt overruns and lost characters in the serial I/O (sio) driver, and any shortage of character list buffers in the tty driver:
23:59:44 ovsiohw/s ovsiodma/s ovclist/s 23:59:49 0.00 0.00 0.00 23:59:54 0.00 0.00 0.00 23:59:59 0.00 0.00 0.00Average 0.00 0.00 0.00
ovsiohw/s shows the number of interrupt overruns per second.
If this value is greater than zero and your system
supports many users logged in over serial lines, you should
consider upgrading to intelligent multiport cards or network
terminal concentrators.
Alternatively, reduce the number of interrupts
generated per incoming character by increasing the interrupt
trigger level on all serial ports that are connected to terminals.
ovsiodma/s shows the number of times per second
that the serial driver lost input characters because there
was insufficient space in the receiver cache.
This is most likely to occur on a serial port receiving
incoming data from a modem.
If the value of ovsiodma/s is greater than zero,
reduce the interrupt trigger level on serial ports that are
connected to modems, and increase the values of
NCLIST and TTHOG.
ovclist/s shows the number of times per second that
the serial driver ran out of character list buffers.
If this value is greater than zero, examine the number of these
buffers using the command:
getconf KERNEL_CLISTS
Increase the current number of character list buffers using:
setconf KERNEL_CLISTS number
until ovclist/s drops back to zero. Use
configure(ADM)
to change NCLIST to this new value.
The following table is a summary of the commands that can be used to view terminal and serial driver activity:
Viewing serial and terminal driver activity
| Command | Field | Description |
|---|---|---|
| [mp]sar -y | rawch/s | number of characters per second handled on the raw input queue |
| rcvin/s | number of interrupts per second notifying that hardware has received input | |
| mdmin/s | number of modem interrupts per second | |
| [mp]sar -g | ovsiohw/s | number of serial driver interrupt overruns per second |
| ovsiodma/s | number of times per second that the serial driver lost input characters | |
| ovclist/s | number of times per second that the system ran out of character list buffers |