net: add benchmark test for SendFile
net: add benchmark test for SendFile

BenchmarkSendFile/1024-4                  716760              1582 ns/op         647.31 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/2048-4                  624632              2057 ns/op         995.55 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/4096-4                  429459              2665 ns/op        1537.06 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/8192-4                  275941              4996 ns/op        1639.56 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/16384-4                 149577              8522 ns/op        1922.58 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/32768-4                  75427             14791 ns/op        2215.47 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/65536-4                  43912             28402 ns/op        2307.41 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/131072-4                 23509             48657 ns/op        2693.82 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/262144-4                 12512             91791 ns/op        2855.87 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/524288-4                  5760            579891 ns/op         904.12 MB/s           0 B/op          0 allocs/op
BenchmarkSendFile/1048576-4                 3020            344129 ns/op        3047.05 MB/s           0 B/op          0 allocs/op

Change-Id: Iaf75e19c9ae8d77b89b3eabb2e9142f59c124536
Reviewed-on: https://go-review.googlesource.com/c/go/+/425878
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Run-TryBot: Andy Pan <e1a1450fb68401f791397dc71aecc2069ecda608@gmail.com>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go