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why adjusting socket buffer boosts PPS?

pw flag

I am using iperf3 to benchmark NIC (82599ES) performance (single port).

  • Ubuntu 18.04 with a self compiled 5.14.14 (from upstream stable git repo)
  • iperf3 3.1.3
  • 82599ES and jumbo frame enabled on switch
  • kernel parameters are more or less default

The command I am running is

# iperf3 -c <IP> -u -l 8000 -t 600 -i 1 -b 10G
[ ID] Interval           Transfer     Bandwidth       Total Datagrams
[  4]   0.00-1.00   sec   415 MBytes  3.48 Gbits/sec  54429  
[  4]   1.00-2.00   sec   414 MBytes  3.47 Gbits/sec  54274  

without any optimization, looks like UDP cannot reach line rate

I then try to increase socket buffer size,

# sysctl -w net.core.wmem_default=2129920
# sysctl -w net.core.wmem_max=2129920

Then running iperf3 with the same options could reach line rate

# iperf3 -c 100.200.116.81  -t 600  -b 10G -i 1 -l 8000 -u
[ ID] Interval           Transfer     Bandwidth       Total Datagrams
[  4]   0.00-1.00   sec  1.15 GBytes  9.91 Gbits/sec  154798  
[  4]   1.00-2.00   sec  1.15 GBytes  9.91 Gbits/sec  154806 

My question is why does increase socket buffer boosts tx performance?

mangohost

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