performance issue when linux(ubuntu) work as router with ip_forwarding enable

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I'm running a test with this topo: tester---(port1)ubuntu_linux(port2)----tester. I enabled ip_forwarding in ubuntu_linux by : echo 1 > /proc/sys/net/ipv4/ip_forward

The tester is trying to find the highest session setup rate, the basic session is: create a tcp port80 session, download a 1 byte file then close the session. The tester is trying to see how high the session setup rate could be.

In my test, the tester shows it reaches 80k sessions/sec, overall traffic throughput is low, only 200Mbps bi-direction. when try higher rate, packets dropping start to happen.

I also did a throughput test, with hundreds session, each session download a big file. overall traffic throughput could reach 2Gbps uni-direction.

To me, it looks like the ubuntu_linux has some limitation on session setup rate. I checked CPU/Memory but cannot find anything reached the limit.

My understanding is: linux does not keep session, only forward packets. So, I was expecting I got similar throughput for setup-rate test and throughput test.

Anybody know why? thanks in advance!

Tried to google "linux ip forwarding session setup rate", but cannot find anywhere talking about this.

hope to know what limits linux ip-forwarding session setup rate. how to improve

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