What do the write_concurrency and read_concurrency ETS options optimize for?

By default, an ETS table uses a locking scheme tuned for a single, simple access pattern. The write_concurrency and read_concurrency options let you tell the table engine what kind of concurrent access to expect, so it can pick internal locking granularity accordingly.

ets:new(my_table, [set, public,
                    {read_concurrency, true},
                    {write_concurrency, true}]).

• read_concurrency — optimizes for many processes reading concurrently, at a small cost to single-reader latency and to switching between heavy reads and heavy writes.
• write_concurrency — splits the table's internal locks more finely so concurrent writers to different keys don't contend with each other, at the cost of slightly higher memory overhead.

Neither option is free: enabling both gives the best concurrent throughput for a table hit hard from many directions at once, but for a table with light or single-process access, the extra locking machinery is pure overhead with no benefit.

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