Category: Networking

TTY Background causing high CPU load

johniLeave a comment

Using an old switch cisco C3548XL switch the CPU load was increasing gradually over a few months.  Looking at the output of show proc the TTY Background process was usings 30-40% CPU load and the system overall was at 80-90%.

This caused some increased ping times, but, switching to the other machines on the switch was fine.  Best solution is to schedule some downtime and reboot… also, possibly remove the “logging synchronous” statement under the line con 0 interface.

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From the post:

TTY Background

The TTY Background process is a generic process used by all terminal
lines (console, aux, async, and so on).
Normally there should not be any impact on the performance of the
router since this process has a lower priority
compared to the other processes that need to be scheduled by the Cisco
IOS software.

If this process takes high CPU utilization, check whether “logging
synchronous” is configured under “line con 0”.
One possible cause could be CSCdy01705 (registered customers only) .

CSCdy01705 Bug Details

Headline High cpu at process TTY Background
Product all Model
Component os Duplicate of
Severity 3 Severity help Status Resolved Status help
First Found-in Version 12.2(10A) All affected versions
First Fixed-in Version 12.3(1.6), 12.3(1.6)T, 12.2(11)T09, 12.3(2.3)B,
12.2(21), 12.2(21.6), 12.2(21.5)S, 12.2(21.7)S,
12.1(21.3)E04, 12.3(7)XI, 12.2(20)S05, 12.1(21.4)EC, 12.2(1S06,
12.2(15)BC02e Version help
Release Notes

A Cisco router may experience high cpu utilization at process TTY
Background when the command “logging synchronous”
is configured under line con 0.

The possible workaround is to remove the command “logging synchronous”
from line con 0, however, this should only be performed during a scheduled
maintenance window as the router could pause indefinitely just after
removal of the command, possibly requiring a manual reboot of the router.