plantegg: 举三反一 (Deriving One from Three)

Core Method

Two learning approaches:

• Knowledge efficiency: Theoretical understanding → intuitive application
• Engineering efficiency: Repeated practice + case studies to reinforce theory

Most people need concrete examples to deeply understand theoretical concepts.

TCP CLOSE_WAIT Diagnosis Example

Problem

Server showed CLOSE_WAIT count equal to somaxconn. Adjusting somaxconn changed the count accordingly.

Theory Application

CLOSE_WAIT means the passive closer (server) is waiting for application to call close(). The fact that CLOSE_WAIT = somaxconn suggested no accept() calls were being made either.

Root Cause

OS open files limit was 1024, preventing new connections. TCP handshakes completed (ESTABLISHED) without application involvement, but applications couldn't accept() → close().

Key Insight

"三次握手成功就变成 ESTABLISHED 了，不需要用户态来accept"

TCP connection state transitions happen in OS kernel. accept() and close() don't affect the kernel-level TCP state machine—they only consume resources after connection is established.