memoize the default shard amount

[Geal]

the default_shard_amount calls std::thread::available_parallelism which makes syscalls to check the resources available in the cgroup. Could this value be stored after the first call? It's unlikely to change often (there could be a mechanism to update it if needed?)

https://github.com/xacrimon/dashmap/blob/459db7ac6f3d0b46f507cb724dd9bb0ce389f4ae/src/lib.rs#L55

[tomkarw]

Made a simple improvement using once_cell, which should do the job.

It's hard for me to think of a case where the value would change and caching the previous result was a meaningful problem. If it really does change, all the previous maps would have the wrong shard amount either way, so I'd rather not overcomplicate it.

[jerry73204]

In the benchmark, it showed that DashMap is heavily hit by the syscall overhead. https://github.com/jerry73204/which-hash-map-is-faster

By running 1000 HashMap initializations per 12 threads, it runs for 57.64 ms. A simple once_cell to cache the shared amount reduces the running time to 1.49ms. That's a great win.

[tomkarw]

@jerry73204 Could you update your benchmarks with results for v5.4.0?