The Linux Implementation of Threads

Linux implements all threads as standard processes and windows has explicit kernel support for threads.

In Linux, a thread is merely a process that shares certain resources (specified by flags) with other processes.

Traditional UNIX scheduler

The priority is exported to user space in the form of nice values, but it leads to several problems.

• Low-priority processes switch very frequently.
• The same priority change will have more effects on lower priorities.
• Timeslice must be some integer multiple of the timer tick.
• Tend to provide interactive processes with more processor time.

Preempt task running in the kernel

The kernel can preempt a task running in the kernel so long as it does not hold a lock.

So we need to add a preemption counter to each process.

The preemption counter increase when lock is acquired and decrease when lock is released.

And the kernel is preemptible when the preemption counter is 0.