netdev - Significant capacity drop on loopback interface

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Date:   Thu, 10 May 2018 15:35:59 +0700
From:   Naruto Nguyen <narutonguyen2018@...il.com>
To:     netdev@...r.kernel.org
Subject: Significant capacity drop on loopback interface

Hello everyone,

Recently, I used netperf to test the TCP performance on loopback
interface on my 2 nodes, one is installed kernel 4.4.103 and the other
is 3.12.61

netperf -l 100 -t TCP_RR
netperf -l 100 -t TCP_RR -- -D

In both cases, I see that the throughput on 4.4.103 is about just 1/2
in comparing with 3.12.61 node

# netperf -l 100 -t TCP_RR
MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0
AF_INET to localhost () port 0 AF_INET : first burst 0
Local /Remote
Socket Size   Request  Resp.   Elapsed  Trans.
Send   Recv   Size     Size    Time     Rate
bytes  Bytes  bytes    bytes   secs.    per sec

16384  87380  1        1       100.00   37714.68
16384  87380

netperf -l 100 -t TCP_RR
MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0
AF_INET to localhost () port 0 AF_INET : first burst 0
Local /Remote
Socket Size   Request  Resp.   Elapsed  Trans.
Send   Recv   Size     Size    Time     Rate
bytes  Bytes  bytes    bytes   secs.    per sec

16384  87380  1        1       100.00   64038.41
16384  87380

When running tcpdump to capture all packets in loopback interface, I
see that during 200s capture, the number of packets on loopback of
4.4.103 is double the number of packets in 3.12.61? Could you please
let me know if it can cause the low throughput as above? Do we have
any tuning for TCP on loopback to improve the performace (actually the
low throughput also happens with UDP) or if we have any known
performance issue in 4.4 kernel on loopback?

Thanks a lot,
Brs,
Naruto