Hi Jie Zhou. The MySQL default is to set innodb_buffer_pool_instances to 8 (in MySQL version 5.7 and up) as this is a good starting point for most general use cases. As per the MySQL documentation, this option takes effect only when you set innodb_buffer_pool_size to a size of 1GB or more. The total size you specify is divided among all the buffer pools. For best efficiency, specify a combination of innodb_buffer_pool_instances and innodb_buffer_pool_size so that each buffer pool instance is at least 1GB.

Great write-up! Impact of innodb_stats_on_metadata was completely new to me.
We also evaluated innodb_io_capacity and innodb_read_ahead_threshold at various settings, no measurable benefit for query workloads that I could find either.

We found good results with changing buffer_pool instances and read_io_threads together in our environment:
Cloud storage, analytic queries, concurrent workload, parallel query enabled (Ali Cloud), 64 cores, o_direct.
innodb_buffer_pool_instances (change from 8 to 32)
innodb_read_io_threads (change from 4 to 16)

Under a 100% physical I/O workload this resulted in a 4x speedup. You mileage will vary, test before production usage. Most query workloads are not 100% PIO, so actual benefits likely much lower.

Cheers,
Jim