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Knowledge Base

Latency / Packets drops due to "ipv4_bh queue" drops, causing Latency / BGP neighborship Flap.` (67201)

Last Updated: 2/21/2019Categories: TroubleshootingTotal Views: 152Language:
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Symptoms

- From Edge-CLI, running multiple iterations of "debug.py  --handoff"  should show "Drops" increment
- Example

velocloud :~# debug.py  --handoff
name               qlimit   lockfree   sleeping    wokenup         enq         deq   drops        head        tail       dummy   next    state
natt_1               1024          0          1          0           0           0       0    0xec1e30    0xec1e30    0xec1e30    0x0   NORMAL
hw_decrypt_0         1024          0          0   42224103    70248161    70248160       0    0xed8090    0xed8090    0xed8090    0x0   NORMAL
ipv4_bh              2048          0          1   44838677    82754935    82754929   78056    0xeb9070    0xeb9070    0xeb9070    0x0   NORMAL

- "var/log/edged.stdout", should show "qmon DEADLOCK_CHECK" errors increment.
- Example

2019-01-11T08:40:46.115 [5948 ] qname=ipv4_bh qmon moving to DEADLOCK_CHECK state packets in queue=2047
2019-01-11T08:41:11.117 [5948 ] qname=ipv4_bh qmon recovered from DEADLOCK_CHECK state to NORMAL state, packets in queue=0
2019-01-11T08:50:51.187 [5948 ] qname=ipv4_bh qmon moving to DEADLOCK_CHECK state packets in queue=1221

Purpose

- To resolve latency seen, while server (Behind Edge-1) is accessed from Client (Behind Edge-2). Due to packets dropped in "ipv4_bh queue"
- To mitigate, BGP / OSPF neighborship flap's, again due to  hello packets dropped in "ipv4_bh queue"

Cause

- Issue (Latency / BGP hello drops) is seen because, when ever "dns_ip_compare" function is performing a cache cleanup, edged is seeing a stall in "ipv4_bh" queue, due to insufficient resources, as "dns_ip_compare" function is hogging most CPU cycles.

- Cause can be found, by running below commands and capturing the outputs in time, as issue lasts only for seconds.

1) tail -F /var/log/edged.stdout | grep “qmon mov” &
2) dispcnt -s  dns_ip_cache_entries_cntr -s memb.mod_fast_learning_db_cache_entry_t.obj_cnt -s memb.mod_slow_learning_db_entry_t.obj_cnt
3) perf top

- As seen in below picture, you will see "dns_ip_compare" function taking highest  CPU cycles, causing stall/deadlock in "ipv4_bh" queue, leading to packets drops.

Impact / Risks

- Latency seen, while server (Behind Edge-1) is accessed from Client (Behind Edge-2). Due to packets dropped in "ipv4_bh queue"
- BGP / OSPF neighborship flap's, again due to  hello packets dropped in "ipv4_bh queue"

Resolution

- Fixed in Version-  R321-20181018-GA-27175-28009
- #28009 is the engineering reference

Workaround

- No workaround.

Related Information

- Salesforce case numbers to refer.
1) 181023-26718

Detectable by VMware SkylineTM

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