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SubjectPoorer networking performance in later kernels?
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(Please keep me CC'd to all comments/responses)

I've tried a kernel upgrade from 3.4.2 to 4.4.0 and see a marked drop in networking performance. Nothing was changed on the test systems, other than the kernel itself (and kernel modules). The identical .config used to build the 3.4.2 kernel was brought over into the 4.4.0 kernel source tree, and any configuration differences (e.g. new parameters, etc.) were taken as default values.

The testing was performed on the same actual hardware for both kernel versions (i.e. take the existing 3.4.2 physical setup, simply boot into the (new) kernel and run the same test). The netperf utility was used for benchmarking and the testing was always performed on idle systems.

TCP testing yielded the following results, where the 4.4.0 kernel only got about 1/2 of the throughput:

Recv Send Send Utilization Service Demand
Socket Socket Message Elapsed Send Recv Send Recv
Size Size Size Time Throughput local remote local remote
bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB

3.4.2 13631488 13631488 8952 30.01 9370.29 10.14 6.50 0.709 0.454
4.4.0 13631488 13631488 8952 30.02 5314.03 9.14 14.31 1.127 1.765

SCTP testing yielded the following results, where the 4.4.0 kernel only got about 1/3 of the throughput:

Recv Send Send Utilization Service Demand
Socket Socket Message Elapsed Send Recv Send Recv
Size Size Size Time Throughput local remote local remote
bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB

3.4.2 13631488 13631488 8952 30.00 2306.22 13.87 13.19 3.941 3.747
4.4.0 13631488 13631488 8952 30.01 882.74 16.86 19.14 12.516 14.210

The same tests were performed a multitude of time, and are always consistent (within a few percent). I've also tried playing with various run-time kernel parameters (/proc/sys/kernel/net/...) on the 4.4.0 kernel to alleviate the issue but have had no success at all.

I'm at a loss as to what could possibly account for such a discrepancy...

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