Cisco 3750 – High CPU – TTY Background

Cisco 3750 - High CPU - TTY Background

Working from an old 3750 stack, noticed the SSH session was definitely sluggish, inputting commands and receiving output not responsive at all.
Because I use exec prompt timestamp under all my console/VTY ports, whenever I run show commands it includes some brief info on the devices CPU and time/date settings.


MDF-01#sh int status
Load for five secs: 57%/1%; one minute: 63%; five minutes: 63%
Time source is NTP, 11:12:17.547 EST Wed Jan 20 2016
Port Name Status Vlan Duplex Speed Type
Gi1/0/1 Uplink to RTR connected 254 a-full a-1000 10/100/1000BaseTX

Checking on the CPU processes and history, one process stands out as unusual for using much CPU.

MDF-01#sh proc cpu sorted
Load for five secs: 76%/0%; one minute: 68%; five minutes: 65%
Time source is NTP, 11:24:14.362 EST Wed Jan 20 2016
CPU utilization for five seconds: 76%/0%; one minute: 68%; five minutes: 65%
PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process
37 2507934490 68679384 36516 45.20% 50.41% 50.66% 0 TTY Background
245 9984 1415 7055 18.84% 5.05% 1.39% 2 SSH Process
74 1260867175 270578900 4659 1.43% 1.13% 1.10% 0 RedEarth Tx Mana
4 951951800 42048527 22639 1.27% 1.33% 1.20% 0 Check heaps

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Show proc cpu history [This is the 1 hour summary graph ]

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Cisco bug ID CSCdy01705 [https://tools.cisco.com/bugsearch/bug/CSCdy01705] points to an issue using logging synchronous on the console port. According to Cisco doc, 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”

After disabling logging synchronous under the line console, CPU dropped off immediately.

MDF-01#conf t
MDF-01(config)#line con 0
MDF-01(config-line)#no logg sync
MDF-01(config-line)#do sh proc cpu sort | i TTY
PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process
37 2507975461 68680981 36516 0.00% 0.00% 0.23% 0 TTY Background

Checking the CPU after 20 minutes:

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