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CPU usage of pfex_junos process increases when adding SFP to uplink module of EX4300-32F

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Article ID: KB31605 KB Last Updated: 29 Oct 2018Version: 2.0
Summary:

When adding a small form-factor pluggable (SFP) to the uplink module of EX4300, the CPU usage of pfex_junos process increases. 

Whenever an SFP is added, software linkscan is enabled by default which causes the CPU utilization to go up. 

Symptoms:

root> show system processes extensive no-forwarding

last pid: 29536;  load averages:  0.68,  0.39,  0.36  up 207+01:08:29    03:41:02
147 processes: 4 running, 120 sleeping, 2 zombie, 21 waiting

Mem: 698M Active, 104M Inact, 151M Wired, 381M Cache, 112M Buf, 1531M Free
Swap:


  PID USERNAME    THR PRI NICE   SIZE    RES STATE    TIME   WCPU COMMAND
   10 root          1 171   52     0K    16K RUN    2902.2 55.22% idle
 1159 root          2  44  -52   560M   206M umtx   1833.1 29.69% pfex_junos

root> show chassis hardware detail no-forwarding

Hardware inventory:
Item             Version  Part number  Serial number     Description
Chassis                                TW3716170123      Virtual Chassis
Routing Engine 0 REV 03   650-064274   TW3716170123      EX4300-32F
Routing Engine 1 REV 19   650-044931   PE3716150608      EX4300-48T
FPC 0            REV 03   650-064274   TW3716170123      EX4300-32F
  CPU                     BUILTIN      BUILTIN           FPC CPU
  PIC 0          REV 03   BUILTIN      BUILTIN           32x 1G SFP, 4x 1G/10G SFP/SFP+
    Xcvr 32      REV 01   740-021309   G3G2002975        SFP+-10G-LR
    Xcvr 33      REV 01   740-021309   G2U2007763        SFP+-10G-LR
    Xcvr 34      REV 01   740-021309   G3G2003132        SFP+-10G-LR
    Xcvr 35      REV 01   740-021309   G3G2003088        SFP+-10G-LR​

Solution:

This is a product limitation, because software linkscan is enabled by default, which causes the CPU utilization to go up. There is no way to disable it.

EX4300 uses a combination of hardware and software linkscan.

    BCM.0> link
    LINKscan: Linkscan enabled
    LINKscan:   Software polling interval: 25000 usec
    LINKscan:   Software Port BitMap 0x0000000000000000000000000000000000000000000000000000bc0000000000: xe0-xe3,hg
    LINKscan:   Hardware Port BitMap 0x000000000000000000000000000000000000000000000000000000000003fffe: ge0-ge16
    LINKscan:   Disabled Port BitMap 0x00000000000000000000000000000000000000000000000000ff403ffffc0000: ge17-ge36,xe4-xe12

With 17 1G SFPs in the front panel ports on an unloaded system, the pfex CPU utilization hovers at around .5%. On plugging in an optic to the uplink or the rear 40G ports, thereby starting software linkscan, the utilization goes up to about 11%. Every additional optic plugged in increases the utilization by about 1%.

Modification History:

2018-10-29: Minor edit. Non-technical.

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