(@orcaman and/or @OneOfOne might be interested in this one?)

I'm interested but I can't commit right now, the one idea that comes to my mind would require hacking runtime/hashmap* otherwise we'd have to double hash the keys.

Very interesting task, I'd love to take this one. I think it should definitely be doable for the Go1.10 Milestone. I'll think about the design some more before I can 100% commit to doing this one in due time.

@orcaman What I wanted to do is something like https://github.com/OneOfOne/cmap/tree/master/v2,

I use a slightly modified version of this in my code, but I wanted to do that with sync.Map in a way, but that'd require a lot more runtime knowledge to be able to use runtime/map's hasher directly to do the sharding rather than double hash it.

Sadly I don't have the knowledge nor time to learn the internals of runtime/map right now.

By all means if you can do that, it'd be great or we can discuss it.

// very simple set/get benchmark using cmap, rwmutex map and sync.Map.

BenchmarkCMap/128-8     20000000               105 ns/op               0 B/op          0 allocs/op
BenchmarkCMap/256-8     20000000                88.6 ns/op             0 B/op          0 allocs/op
BenchmarkCMap/512-8     20000000                67.6 ns/op             0 B/op          0 allocs/op
BenchmarkCMap/1024-8    30000000                42.3 ns/op             0 B/op          0 allocs/op
BenchmarkCMap/2048-8    30000000                34.5 ns/op             0 B/op          0 allocs/op
BenchmarkCMap/4096-8    50000000                33.0 ns/op             0 B/op          0 allocs/op
BenchmarkMutexMap-8     10000000               196 ns/op               0 B/op          0 allocs/op
BenchmarkSyncMap-8      30000000                39.3 ns/op            16 B/op          1 allocs/op

I think you can just use a slice of locks and dirty maps with the low order bits of the hash performing the selection - but this requires access to the internal hash code.

@bcmills I am willing to give this a try. Is there a document that desribes using internal facilities when an implementation is part of the stdlib?

I don't know of any good starting docs, but perhaps @randall77 or @aclements can point you in the right direction.

really a good idea