Linux Networking Stack Performance

Here is a link about the TCP/IP stack on Linux 2.4 and 2.6. The key insights are as follows

1. Kernel Locking improvements: For SMP, Linux 2.6 still does use the "Big Kernel Lock" to prevent different CPUs from accessing the kernel at the same time , but it has finer granularity. 
2. Efficient copy routines in 2.6 
3. 2.6 Kernel uses the O(1) scheduler, and post 2.6.23 uses a O(log n) scheduler . See this and this  and this (Info about how sessions are made up of processes and how tty is associated with the session, processes/tasks are made up of threads that share the thread group id, the O(1) algo using 2 runqueues per CPU, task priority, nice values etc.)