Chapter 4 - DSpace at Waseda University

Chapter 5
5.1 Timer Latency
When a process is calling a function of the Linux kernel, it uses a system call. However,
when hardware is calling the Linux kernel, it uses interrupt. When the kernel receives
interrupts it stops its process and operates an interrupt handler. It is clear that a priority is
given to the interrupt. Request of interrupt handler with higher priority will stop the lower
priority task and it will resume when finished the higher priority task. All kernels operate by
interrupt (hardware interrupt or software interrupt). As a timer also operates inside the kernel,
it is also operate by the interrupt. The timer controller will generate interrupts periodically.
Commonly, Linux timer interrupts utilize a global timer interrupt and a local timer interrupt.
Timer latency means to miss deadline. There are two reasons the timer latency. Firstly,
latency arises as there are many required tasks to run after occurrence of interrupt. Figure 5.1
is shown, each latency required from hardware interrupts to be scheduled.
Figure 5.1 Task Preemption Latency Model
� Interrupt Latency: Latency before starting of Interrupt Service Routine (ISR) after
occurred hardware interrupts [30]. Hardware latency, interrupt disable latency,
interrupt vectoring latency, interrupt dispatch latency are included.
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