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28 :122

	English image::foa-metadata-interleaving.png[align=center] 50/10000 · 50
29 :124

	English A more detailed description of Boost.Unordered's open-addressing implementation is given in an https://bannalia.blogspot.com/2022/11/inside-boostunorderedflatmap.html[external article]. For more information on implementation rationale, read the xref:rationale.adoc#rationale_open_addresing_containers[corresponding section]. 324/10000 · 324
30 :130

	English Concurrent Containers 21/10000 · 21
31 :132

	English `boost::concurrent_flat_set`/`boost::concurrent_node_set` and `boost::concurrent_flat_map`/`boost::concurrent_node_map` use the basic xref:structures.adoc#structures_open_addressing_containers[open-addressing layout] described above augmented with synchronization mechanisms. 275/10000 · 275
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	English image::cfoa.png[align=center] 29/10000 · 29
33 :142

	English Two levels of synchronization are used: 39/10000 · 39
34 :144

	English Container level: A read-write mutex is used to control access from any operation 80/10000 · 80
35 :145

	English to the container. Typically, such access is in read mode (that is, concurrent) even for modifying operations, so for most practical purposes there is no thread contention at this level. Access is only in write mode (blocking) when rehashing or performing container-wide operations such as swapping or assignment. 312/10000 · 312
36 :149

	English Group level: Each 15-slot group is equipped with an 8-byte word containing: 75/10000 · 75
37 :150

	English A read-write spinlock for synchronized access to any element in the group. An atomic _insertion counter_ used for optimistic insertion as described below. 160/10000 · 160
38 :154

	English By using atomic operations to access the group metadata, lookup is (group-level) lock-free up to the point where an actual comparison needs to be done with an element that has been previously SIMD-matched: only then is the group's spinlock used. 245/10000 · 245
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	English Insertion uses the following _optimistic algorithm_: 52/10000 · 52
40 :160

	English The value of the insertion counter for the initial group in the probe 69/10000 · 69
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	English sequence is locally recorded (let's call this value `c0`). 58/10000 · 58
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	English Lookup is as described above. If lookup finds no equivalent element, 68/10000 · 68
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	English search for an available slot for insertion successively locks/unlocks each group in the probing sequence. 105/10000 · 105
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	English When an available slot is located, it is preemptively occupied (its 67/10000 · 67
45 :166

	English reduced hash value is set) and the insertion counter is atomically incremented: if no other thread has incremented the counter during the whole operation (which is checked by comparing with `c0`), then we're good to go and complete the insertion, otherwise we roll back and start over. 285/10000 · 285
46 :172

	English This algorithm has very low contention both at the lookup and actual insertion phases in exchange for the possibility that computations have to be started over if some other thread interferes in the process by performing a succesful insertion beginning at the same group. In practice, the start-over frequency is extremely small, measured in the range of parts per million for some of our benchmarks. 400/10000 · 400
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	English For more information on implementation rationale, read the xref:rationale.adoc#rationale_concurrent_containers[corresponding section]. 134/10000 · 134
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