Why Does My Nginx + PHP-FPM Seem Single-Threaded? Unmasking the PHP Session Lock

Have you ever noticed that a long-running PHP request blocks all other requests from the same user, making your high-performance Nginx server appear to be single-threaded? This isn't an Nginx issue. This article dives deep into the root cause—PHP's default session file locking mechanism—and provides three effective solutions, including the best practice `session_write_close()`, to eliminate concurrency bottlenecks and unleash your server's full potential.