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added implementation rationale for concurrent hashmap

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joaquintides
2023-05-13 19:29:08 +02:00
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doc/unordered/rationale.adoc
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@ -108,7 +108,7 @@ When using a hash function directly suitable for open addressing, post-mixing ca
=== Platform interoperability
The observable behavior of `boost::unordered_flat_set`/`unordered_node_set` and `boost::unordered_flat_map`/`unordered_node_map` is deterministically
identical across different compilers as long as their ``std::size_type``s are the same size and the user-provided
identical across different compilers as long as their ``std::size_t``s are the same size and the user-provided
hash function and equality predicate are also interoperable
—this includes elements being ordered in exactly the same way for the same sequence of
operations.
@ -117,3 +117,25 @@ Although the implementation internally uses SIMD technologies, such as https://e
and https://en.wikipedia.org/wiki/ARM_architecture_family#Advanced_SIMD_(NEON)[Neon^], when available,
this does not affect interoperatility. For instance, the behavior is the same
for Visual Studio on an x64-mode Intel CPU with SSE2 and for GCC on an IBM s390x without any supported SIMD technology.
== Concurrent Hashmap
The same data structure used by Boost.Unordered open-addressing containers has been chosen
also as the foundation of `boost::concurrent_flat_map`:
* Open-addressing is faster than closed-addressing alternatives, both in non-concurrent and
concurrent scenarios.
* Open-addressing layouts are eminently suitable for concurrent access and modification
with minimal locking. In particular, the metadata array can be used for implementations of
lookup that are lock-free up to the last step of actual element comparison.
* Layout compatibility with Boost.Unordered flat containers allows for fast transfer
of all elements between `boost::concurrent_flat_map` and `boost::unordered_flat_map`.
(This feature has not been implemented yet.)
=== Hash function and platform interopersability
`boost::concurrent_flat_map` makes the same decisions and provides the same guarantees
as Boost.Unordered open-addressing containers with regards to
xref:#rationale_hash_function[hash function defaults] and
xref:#rationale_platform_interoperability[platform interoperability].