Debugging MySQL Memory Alerts: When innodb_buffer_pool_size Isn't the Whole Story

A deep debugging walkthrough of a MySQL RSS memory mystery on a GCP c4d-standard-4 instance where mysqld consumed 10.4 GB despite a 7168 MB innodb_buffer_pool_size. The gap is traced through four layers: expected InnoDB/Performance Schema/thread-buffer overhead, ~9% mmap chunk padding on buffer pool allocations, jemalloc dirty pages accumulating on an idle DR replica, and the root cause - MALLOC_CONF not set, leaving jemalloc's background_thread decay disabled. Fixes include lowering the buffer pool to a clean chunk multiple (6144 MB), cutting max_connections from 3000 to 500, enabling jemalloc's background_thread with a 5-second decay, and adding swap for OOM headroom, bringing RSS down to ~7.9 GB.

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