InnoDB dynamically calculates the size of the read-ahead area in case it has to trigger its read-ahead algorithm. When the workload involves heavy I/O operations, this size is computed so frequently that it has a non-negligeable impact on the CPU usage.
This variable only depends on the size of the buffer pool set by the innodb_buffer_pool_size variable, and as soon as the buffer pool has a size properly greater than 1024 pages (or 16 MB), it is always 64. With this change, its value is fixed to 64, thus removing a bottleneck experienced by some users.
Please note that the minimum allowed value for the InnoDB buffer pool is de facto set to 32 MB.
This change is a port of the feature from Facebook: