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Knowledge Base

CPU usage on AR161

Publication Date:  2017-03-30  |   Views:  330  |   Downloads:  0  |   Author:  SU1002255490  |   Document ID:  EKB1000570053

Contents

Issue Description

During tests of AR158E and AR161, it was noticed that router AR161 behaves strange. Both routers were configured in the same way.

For traffic level of 80 Mbps, CPU usage of AR158E was 56%, but on AR161, the CPU usage was larger 70%. 

After a couple of more tests were done, for higher amount of traffic, the results were quite strange because with increase of traffic, the CPU usage should increase too, linearly.

On AR161 in the range of 70M to 90M CPU usage increases linearly:

CPU usage

Traffic (Mbps)

62.8%

70M

71.6%

80M

73.5%

90M

But on the same AR161 in the range of 100M to 110M CPU usage decreases linearly: 

CPU usage

Traffic (Mbps)

65.2%

100M

64.3%

110M

Next in the range of 120M to 230M CPU usage increases linearly: 

CPU usage

Traffic (Mbps)

65.1%

120M

68.1%

130M

70.1%

140M

73.1%

150M

75.8%

160M

At 230M, CPU usage is equal to 100%.

Solution

The AR150/160 routers use NAPI to receive packets from the network. The total CPU usage is composed by two parts:

1)     Interrupts of the input packets

2)     Other services to deal with the packet’s forwarding

At first, the CPU usage is accelerating with the throughput of traffic, but when the traffic grows, the interrupts used by the NAPI drop, because each interrupt can deal with more packets.

Lab test results are as follows:

AR160:


AR150: