Understanding the Linux Kernel

10.4. Anticipating Linux 2.4

Linux 2.4 introduces a subtle optimization concerning TLB flushing for kernel threads and zombie processes. As a result, the active Page Global Directory is set by the schedule( ) function rather than by the switch_to macro.

The Linux 2.4 scheduling algorithm for SMP machines has been improved and simplified. Whenever a new process becomes runnable, the kernel checks whether the preferred CPU of the process, that is, the CPU on which it was last running, is idle; in this case, the kernel assigns the process to that CPU. Otherwise, the kernel assigns the process to another idle CPU, if any. If all CPUs are busy, the kernel checks whether the process has enough priority to preempt the process running on the preferred CPU. If not, the kernel tries to preempt some other CPU only if the new runnable process is real-time or if it has short average time slices compared to the hardware cache rewriting time. (Roughly, preemption occurs if the new runnable process is interactive and the preferred CPU will not reschedule shortly.)