sync: reduce contention between Map operations with new-but-disjoint keys

[bcmills]

The Go 1.9 implementation of sync.Map uses a single Mutex to guard the read-write map containing new keys. That makes Store calls with different new keys always contend with each other, and also contend with Load calls with different new keys, even if the Loads and Stores for each key are confined to a single thread.

That doesn't really matter for the sync.Map use-cases in the standard library because they do not operate on new keys in the steady state, but it limits the utility of sync.Map for use-cases involving a high rate of churn. Such use-cases may include:

• caches with high eviction rates, such as caches fronting key-value storage services
• maps of RPC or HTTP stream ID to handler state
• maps of opaque handles to Go pointers in order to write C-exported APIs that comply with cgo pointer-passing rules

We should explore ways to address new-key contention, such as sharding the read-write maps and associated locks (as suggested in #20360), journaling writes (and using a Bloom or HyperLogLog filter to avoid reading the journal, along the lines of #21032), or storing the read-write map in an atomic tree data structure instead of a built-in map.

[bcmills]

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

[OneOfOne]

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.

[orcaman]

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.

[OneOfOne]

@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

[robaho]

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.

[robaho]

@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?