Bug 2128446

(In reply to Li Tian from comment #0)
> Description of problem:
> UDP data loss is supposed be less then 1% in the below situation.
> 1. vRSS hashing excludes the port where UDP data is through.
> # ethtool -n eth0 rx-flow-hash udp4
> UDP over IPV4 flows use these fields for computing Hash flow key:
> IP SA
> IP DA
> 2. Packets length is larger than MTU, e.g. 8k.
> 
> Tested with 2 Standard_D15_v2 VMs using the below commands on each
> respectively:
> # iperf3 -s 4 -p 8001
> # iperf3 -u -c 10.0.0.4 -p 8001 -4 -b 0 -l 8k -P 64 -t 60
> --get-server-output -i 60
> 
> And the result is:
> [ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total
> Datagrams
> [  5]   0.00-60.01  sec  1.09 MBytes   152 Kbits/sec  4.741 ms  31221/31360
> (1e+02%)  receiver
> [  6]   0.00-60.01  sec   944 KBytes   129 Kbits/sec  3.708 ms  30642/30760
> (1e+02%)  receiver
> ...
> [SUM]   0.00-60.01  sec  55.2 MBytes  7.71 Mbits/sec  5.510 ms 
> 1646217/1653281 (1e+02%)  receiver
> 
> Version-Release number of selected component (if applicable):
> 4.18.0-372.29.1.el8_6.x86_64
> 
> How reproducible:
> 100% on Azure.
> 
> Steps to Reproduce:
> 1. Create 2 VMs and disable vRSS hashing by - 
> # ethtool -N eth0 rx-flow-hash udp4 sd
> 2. run iperf3 commands in description individually.
> 
> Actual results:
> Data loss is greater than 1%.
> 
> Expected results:
> Data loss is less than 1%.
> 
> Additional info:
> 1. This issue presents from 8.6 through 9.1.
> 2. Related BZ - https://bugzilla.redhat.com/show_bug.cgi?id=1474300 (looks
> like a re-occurrence).
> 3. Issue does not present when packets are smaller than MTU.
> 4. Issue presents on other VM sizes, e.g. Standard_D16s_v5.

Is it a realistic scenario in real world to have package size larger than the MTU setting?

@vkuznets perhaps you can shed a light on this matter?

Thanks!

(In reply to Eduardo Otubo from comment #1)
> 
> Is it a realistic scenario in real world to have package size larger than
> the MTU setting?
> 

Generally speaking it is. An application using UDP has no idea about the MTU
size of the underlying NIC, it can always try sending bigger packets. I'm not
sure how common it is though.

Issue still presents on latest 8.8 (4.18.0-472.el8.x86_64):

[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-60.02  sec   584 KBytes  79.7 Kbits/sec  0.111 ms  69556/69629 (1e+02%)  receiver
[  6]   0.00-60.02  sec   600 KBytes  81.9 Kbits/sec  0.072 ms  69521/69596 (1e+02%)  receiver
...
[SUM]   0.00-60.02  sec  40.4 MBytes  5.65 Mbits/sec  0.135 ms  4450502/4455678 (1e+02%)  receiver

'1. This issue presents from 8.6 through 9.1.'
--- What's the result for 8.5/9.0?

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