net/http: re-simplify HTTP/1.x status line writing
It used to be simple, and then it got complicated for speed (to reduce
allocations, mostly), but that involved a mutex and hurt multi-core
performance, contending on the mutex.
A change was sent to try to improve that mutex contention in
https://go-review.googlesource.com/c/42110/2/src/net/http/server.go
but that introduced its own allocations (the string->interface{}
boxing for the sync.Map key), which runs counter to the whole point of
that statusLine function: to remove allocations.
Instead, make the code simple again and not have a mutex. It's a bit
slower for the single-core case, but nobody with a single-user HTTP
server cares about 50 nanoseconds:
name old alloc/op new alloc/op delta
ResponseStatusLine 0.00B ±NaN% 0.00B ±NaN% ~ (all samples are equal)
ResponseStatusLine-2 0.00B ±NaN% 0.00B ±NaN% ~ (all samples are equal)
ResponseStatusLine-4 0.00B ±NaN% 0.00B ±NaN% ~ (all samples are equal)
name old allocs/op new allocs/op delta
ResponseStatusLine 0.00 ±NaN% 0.00 ±NaN% ~ (all samples are equal)
ResponseStatusLine-2 0.00 ±NaN% 0.00 ±NaN% ~ (all samples are equal)
ResponseStatusLine-4 0.00 ±NaN% 0.00 ±NaN% ~ (all samples are equal)
(Note the code could be even simpler with fmt.Fprintf, but that is
relatively slow and involves a bunch of allocations getting arguments
into interface{} for the call)