In computing, interrupt latency refers to the delay between the start of an Interrupt Request (IRQ) and the start of the respective Interrupt Service Routine (ISR). For many operating systems, devices are serviced as soon as the device's

interrupt handler In computer systems programming, an interrupt handler, also known as an interrupt service routine or ISR, is a special block of code associated with a specific interrupt condition. Interrupt handlers are initiated by hardware interrupts, softwar ...

is executed. Interrupt latency may be affected by

microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...

design, interrupt controllers, interrupt masking, and the

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...

's (OS) interrupt handling methods.

Background

There is usually a trade-off between interrupt latency,

throughput Network throughput (or just throughput, when in context) refers to the rate of message delivery over a communication channel, such as Ethernet or packet radio, in a communication network. The data that these messages contain may be delivered ov ...

, and processor utilization. Many of the techniques of CPU and OS design that improve interrupt latency will decrease throughput and increase processor utilization. Techniques that increase throughput may increase interrupt latency and increase processor utilization. Lastly, trying to reduce processor utilization may increase interrupt latency and decrease throughput. Minimum interrupt latency is largely determined by the interrupt controller circuit and its configuration. They can also affect the

jitter In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significa ...

in the interrupt latency, which can drastically affect the

real-time Real-time or real time describes various operations in computing or other processes that must guarantee response times within a specified time (deadline), usually a relatively short time. A real-time process is generally one that happens in defined ...

schedulability of the system. The

Intel APIC architecture In computing, Intel's Advanced Programmable Interrupt Controller (APIC) is a family of interrupt controllers. As its name suggests, the APIC is more advanced than Intel's 8259 Programmable Interrupt Controller (PIC), particularly enabling the co ...

is well known for producing a huge amount of interrupt latency jitter. Maximum interrupt latency is largely determined by the methods an OS uses for interrupt handling. For example, most processors allow programs to disable interrupts, putting off the execution of interrupt handlers, in order to protect

critical section In concurrent programming, concurrent accesses to shared resources can lead to unexpected or erroneous behavior, so parts of the program where the shared resource is accessed need to be protected in ways that avoid the concurrent access. One way to ...

s of code. During the execution of such a critical section, all interrupt handlers that cannot execute safely within a critical section are blocked (they save the minimum amount of information required to restart the interrupt handler after all critical sections have exited). So the interrupt latency for a blocked interrupt is extended to the end of the critical section, plus any interrupts with equal and higher priority that arrived while the block was in place. Many computer systems require low interrupt latencies, especially

embedded system An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' as ...

s that need to

control Control may refer to: Basic meanings Economics and business * Control (management), an element of management * Control, an element of management accounting * Comptroller (or controller), a senior financial officer in an organization * Controllin ...

machinery in real-time. Sometimes these systems use a

real-time operating system A real-time operating system (RTOS) is an operating system (OS) for real-time applications that processes data and events that have critically defined time constraints. An RTOS is distinct from a time-sharing operating system, such as Unix, which m ...

(RTOS). An RTOS makes the promise that no more than a specified maximum amount of time will pass between executions of

subroutine In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Functions may ...

s. In order to do this, the RTOS must also guarantee that interrupt latency will never exceed a predefined maximum.

Considerations

Advanced interrupt controllers implement a multitude of hardware features in order to minimize the overhead during

context switch In computing, a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point, and then restoring a different, previously saved, state. This allows multiple processes ...

es and the effective interrupt latency. These include features like: * Minimum jitter through non-interruptible instructions * Zero wait states for the memory system * Switchable register banks * Tail chaining * Lazy stacking * Late arrival * Pop preemption * Sleep-on-exit feature Also, there are many other methods hardware may use to help lower the requirements for shorter interrupt latency in order to make a given interrupt latency tolerable in a situation. These include buffers, and flow control. For example, most network cards implement transmit and receive

ring buffer In computer science, a circular buffer, circular queue, cyclic buffer or ring buffer is a data structure that uses a single, fixed-size buffer as if it were connected end-to-end. This structure lends itself easily to buffering data streams. Ther ...

s, interrupt rate limiting, and hardware flow control. Buffers allow data to be stored until it can be transferred, and flow control allows the network card to pause communications without having to discard data if the buffer is full. Modern hardware also implements interrupt rate limiting. This helps prevent

interrupt storm In operating systems, an interrupt storm is an event during which a processor receives an inordinate number of interrupts that consume the majority of the processor's time. Interrupt storms are typically caused by hardware devices that do not suppo ...

s or

live-lock In concurrent computing, deadlock is any situation in which no member of some group of entities can proceed because each waits for another member, including itself, to take action, such as sending a message or, more commonly, releasing a loc ...

s by having the hardware wait a programmable minimum amount of time between each interrupt it generates. Interrupt rate limiting reduces the amount of time spent servicing interrupts, allowing the processor to spend more time doing useful work. Exceeding this time results in a soft (recoverable) or hard (non-recoverable) error.

References

{{DEFAULTSORT:Interrupt Latency Latency

Background

Considerations

See also

References