Decentralized counters for ETS ordered_set with write_concurrency #2190
Previously, all ETS tables used centralized counter variables to keep
The new counter is implemented in the module erts_flxctr
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Previously, all ETS tables used centralized counter variables to keep track of the number of items stored and the amount of memory consumed. These counters can cause scalability problems (especially on big NUMA systems). This commit adds an implementation of a decentralized counter and modifies the implementation of ETS so that ETS tables of type ordered_set with write_concurrency enabled use the decentralized counter. [Experiments] indicate that this change substantially improves the scalability of ETS ordered_set tables with write_concurrency enabled in scenarios with frequent `ets:insert/2` and `ets:delete/2` calls. The new counter is implemented in the module erts_flxctr (`erts_flxctr.h` and `erts_flxctr.c`). The module has the suffix flxctr as it contains the implementation of a flexible counter (i.e., counter instances can be configured to be either centralized or decentralized). Counters that are configured to be centralized are implemented with a single counter variable which is modified with atomic operations. Decentralized counters are spread over several cache lines (how many can be configured with the parameter `+dcg`). The scheduler threads are mapped to cache lines so that there is no single point of contention when decentralized counters are updated. The thread progress functionality of the Erlang VM is utilized to implement support for linearizable snapshots of decentralized counters. The snapshot functionality is used by the `ets:info/1` and `ets:info/2` functions. : http://winsh.me/ets_catree_benchmark/flxctr_res.html
This commit refactors a function to increase readability and adds a test case to increase the test coverage.