runtime, net: the series of BenchmarkTCP fail from Go 1.5 when GOMAXPROCS is greater than # of CPU cores

[mikioh]

For example,

On Linux:

go test -v -run=none -bench=TCP -benchmem
PASS
BenchmarkTCP4OneShot-4                 30000       4566099 ns/op        1916 B/op         25 allocs/op
--- BENCH: BenchmarkTCP4OneShot-4
    tcp_test.go:138: dial tcp 127.0.0.1:34889: getsockopt: connection timed out
    tcp_test.go:138: dial tcp 127.0.0.1:34889: getsockopt: connection timed out
    tcp_test.go:138: dial tcp 127.0.0.1:34889: getsockopt: connection timed out
BenchmarkTCP4OneShotTimeout-4          20000         74035 ns/op        1902 B/op

Epoll or runtime.netpoll missed some write events?

On Darwin:

go test -v -run=none -bench=TCP -benchmem
PASS
BenchmarkTCP4OneShot-8                 10000        142858 ns/op        2123 B/op         25 allocs/op
--- BENCH: BenchmarkTCP4OneShot-8
    tcp_test.go:80: write tcp 127.0.0.1:59676->127.0.0.1:56708: write: socket is not connected
    tcp_test.go:80: write tcp 127.0.0.1:49711->127.0.0.1:56708: write: socket is not connected
BenchmarkTCP4OneShotTimeout-8           3000        360687 ns/op        2108 B/op         25 allocs/op

Kqueue or runtime.netpoll misrecognized writable sockets?

[mikioh]

Luckily this doesn't happen on Linux with ee566d5.

Also just scratched OS X 10.11 using dtrace and confirmed that the write syscall returns ENOTCONN certainly. Probably it's a kernel bug. Closing.

BenchmarkTCP4OneShot-8                 10000        142858 ns/op        2123 B/op         25 allocs/op
--- BENCH: BenchmarkTCP4OneShot-8
    tcp_test.go:80: s=27 write tcp 127.0.0.1:63425->127.0.0.1:60520: write: socket is not connected

sudo ./syscall.d 
dtrace: script './syscall.d' matched 10 probes
CPU     ID                    FUNCTION:NAME
  0    337                    connect:entry tid=449416 s=27
  0    338                   connect:return tid=449416 s=27 errno=36
  0    377                 getsockopt:entry tid=449419 s=27
  0    378                getsockopt:return tid=449419 s=27 errno=0
  0    149                      write:entry tid=449419 s=27
  0    150                     write:return tid=449419 s=27 errno=57