When the window is not fully open, your TCP stack is doing more than you think

This post delves into the complex memory and window management of the TCP receiving side within the Linux kernel. It clarifies how Linux manages TCP receive buffers and windows, answering questions about the actual capacity of receive buffers and the speed at which they can be filled. The post explains the concepts of skmem_rb, recv-q, skmem_r, and the advertised window, detailing how settings like tcp_adv_win_scale influence performance. It highlights the critical issue of exceeding memory budgets, leading to packet drops and performance degradation, and discusses the roles of TCP Coalesce and TCP Collapse in mitigating these problems.