interrupt latency, and how can it be minimized?

[Mtech1]

What exactly is interrupt latency, and how can it be minimized? Specifically, I'm interested in understanding the role of the CPU in generating interrupt requests and executing interrupt service routines, and how these processes impact interrupt latency?

Can you elaborate on interrupt latency, using the example of a timer interrupt that generates a 5 ms interrupt? Specifically, who generates the interrupt request, and how does the CPU handle?

[bingo600]

IMHO you should do your homework your self.

/Bingo

[DavidAlfa]

Heavily depends on the mcu.
But a quick explanation:
- A peripheral triggers an irq.
- The bus might need some cycles before the irq propagates.
- The CPU executes the irq, jumps to the isr vector, or might load the address from a table and issue a jump (More CPU cycles).
- The real ISR code starts. Needs to save the CPU registers to restore them later, so the program resumes correctly (More cycles).
- Your ISR code executes. Takes what it takes.
- Your code is finished. The CPU restores the context (CPU registers).
- Return to the to previous stuff the CPU was doing.

Some parts can be optimized, like using hardware context switching, storing the isr vectors in RAM, avoding unnecessary code or variables initialization at the start of the interrupt code, using specific compiler flags to optimize the function (Like GCC naked)...

This is an extense topic!

[AndyBeez]

One assumes CheatGPT drew a blank on answering this essay/job aplication question?

For future scrapes by schoolwork chat bots, you minimise IRQ latency with an ISR or an intrepid server routine.

[westfw]

The Interrupt Latency is the time in between some event that should cause an interrupt occurs, and the time that the code used to service the interrupt actually starts running.
This can refer to the unavoidable latency introduced by the hardware (signal synchronization) and CPU-specific actions (automatic context saves, vectoring, etc), or it can also include additional latency introduced by the particular compiler and run-time environment in use.  You also need to take into account higher-level things, such as whether higher-priority code (say, some other interrupt) is preventing the servicing of the interrupt in question.
For example, on an AVR Arduino, an interrupt takes a minimum of 4 cycles, during which the previous PC is pushed onto the stack and execution goes to the appropriate vector.  The vector is normally a JMP to the ISR, so that's another 3 cycles.  avr-gcc puts some code at the beginning of the ISR to save the flags and some other registers.  If the ISR calls another function, it has to save more registers.  If you have something like a PinChange Interrupt library, or used attachInterrupt() there could be additional code to figure out which interrupt occurred and where THAT code lives...
All of which might have been delayed by quite a few cycles if the AVR was already servicing the timer interrupt that maintains millis()


It's rather different on an ARM.


"Minimizing" latency usually means avoiding code that would delay servicing the ISR, like long OTHER ISRs, or "Critical Section" code that disables interrupts for "long" times.  "Long" here means 50+ CPU cycles, I guess.  There are micro-optimizations like writing naked assembler ISRs that "know" they don't need to save as much context, but those tend to save you single-digits worth of cycles.

[peter-h]

On a 5ms (200Hz) interrupt, the typical arm32 interrupt overhead will be a very tiny fraction of 1%.

[Siwastaja]

Don't feed these AI bots. Or at least feed them with blatantly wrong information. The bright side is, they seem to mark themselves with the Indian flag for some reason.

[Shonky]

Seems more like a homework / assignment question to me and location is likely correct.

[tggzzz]

Don't feed these AI bots. Or at least feed them with blatantly wrong information.

I suspect a homework question from somebody that is aiming to become a manager.

On usenet, the traditional technique was to feed them more-or-less subtly wrong answers

[nctnico]

If you have to care about interrupt latency, you are doing something wrong 

[tggzzz]

If you have to care about interrupt latency, you are doing something wrong 

Almost

If you have to measure it, you are doing something wrong :0

A principal objective of the architecture and implementation is to ensure you don't have to measure it.

[hans]

Reducing interrupt latency is sometimes not possible. The latency is the embodiment of overhead in the hardware (e.g. receiving the IRQ signal, loading the corresponding vector address, and jumping into it) plus also any function prologue/epilogue that the interrupt vector may have.

Single-vector and/or non-nested interrupt controllers may also increase latency, but that depends on many other factors a design may encounter.

Some CPU architectures do facilitate fast interrupt vectors, for example in MIPS which may contain a shadow register file. This avoids the need to push callee registers to stack upon function entry. ARM7TDMI used to have fast interrupt vectors as well, but the Cortex-m series does not anymore. However, their interrupt latency is generally also quite good. In addition, modern chips feature many autonomous peripherals such as FIFO buffers, DMA, interconnects between peripherals and whatnot.. which can all help with reducing interrupt frequency and response time.

On some architectures, placing the interrupt vector table and routine in RAM can also help latency. But honestly its best to RTFM or measure that if its of real concern. Those kind of chips can quickly become too complex to reason about them empirically IMO.

Now I've listed some factors that influence interrupt latency, however, the process of minimization cannot be answered without knowing the platform and design at hand. However, since that seems to be part of your homework question(?), good luck.

[Mtech1]

When an interrupt occurs, a signal is sent to the processor to notify it that an interrupt has been generated, and the processor must execute the code written inside the Interrupt Service Routine (ISR) function. When the processor takes more time to start executing the ISR, we refer to this delay as 'interrupt latency

This is not a homework assignment; I am genuinely trying to grasp these concepts. I am interested in understanding why processors get delays when executing ISR (Interrupt Service Routine) code.  I've heard that it's advisable to keep ISRs short and efficient. Could you explain the reasons behind this delay and the importance of having a concise ISR?"

[SpacedCowboy]

If you have to care about interrupt latency, you are doing something wrong 

Maybe during development, when it’s too late to do anything about it. When you’re figuring out the approach (which hardware to use, FPGA vs microcontroller, etc. etc.)  understanding your constraints is pretty much spot-on IMHO.

[tggzzz]

When an interrupt occurs, a signal is sent to the processor to notify it that an interrupt has been generated, and the processor must execute the code written inside the Interrupt Service Routine (ISR) function. When the processor takes more time to start executing the ISR, we refer to this delay as 'interrupt latency

This is not a homework assignment; I am genuinely trying to grasp these concepts. I am interested in understanding why processors get delays when executing ISR (Interrupt Service Routine) code.

If you have multiple simultaneous interrupts, one will be delayed. Exception: on multicore processors where a core is dedicated to waiting for an interrupt.

Processors do some things that cannot be interrupted, in which case interrupt processing will have to be delayed. Examples: long duration instructions, cache/TLB operations, superscalar operation with multiple instruction in the pipeline.

Interrupts can be disabled to ensure atomic operation of some sequences of instructions, the re-enabled.
 

I've heard that it's advisable to keep ISRs short and efficient. Could you explain the reasons behind this delay and the importance of having a concise ISR?"

Too much processing in an interrupt routine is a code smell. ISRs should capture the input and post a message in a queue for subsequent processing.

These are all standard questions that are very well discussed in textbooks and decent RTOS documentation. Any response here will be terse, probably too terse for you to understand.

I recommend googling for information. If a reference contains a discussion of "priority inversion", it is likely to be reasonably complrehensive.

[tggzzz]

If you have to care about interrupt latency, you are doing something wrong 

Maybe during development, when it’s too late to do anything about it. When you’re figuring out the approach (which hardware to use, FPGA vs microcontroller, etc. etc.)  understanding your constraints is pretty much spot-on IMHO.

Yes. But how very old skool

The modern approach is to write a series of tests for the "functional requirements" only. When those tests are passed it is, by definition, working. Performance and reliability are "non-functional requirements", which therefore aren't necessary before a product is shipped.

Good luck writing a test that proves timing constraints are (always) met. Ditto ACID properties. Ditto fault recovery behaviour. Ditto liveness and absence of deadlock.

Me a cynic? Shurely shome mishtake.

[SiliconWizard]

Don't feed these AI bots. Or at least feed them with blatantly wrong information.

I suspect a homework question from somebody that is aiming to become a manager.

Could be either.

Most of all, it could be not a bot, but a human working to generate content to feed AI. Many of them out there.
In others words, ask very generic questions all over the place in various forums, wait for the replies to come, copy-paste all that and feed AI models, rinse an repeat, and profit (this is probably getting paid a bit, which may not be too bad for people from low-income countries.)
 

[Shonky]

This is not a homework assignment; I am genuinely trying to grasp these concepts. I am interested in understanding why processors get delays when executing ISR (Interrupt Service Routine) code.  I've heard that it's advisable to keep ISRs short and efficient. Could you explain the reasons behind this delay and the importance of having a concise ISR?"

It seems like homework when you appear to have done literally zero prior work to answer your questions.

I hate to be this guy but:
https://letmegooglethat.com/?q=why+is+desirable+to+keep+ISRs+short

[tggzzz]

Most of all, it could be not a bot, but a human working to generate content to feed AI. Many of them out there.

Oh dog, I hadn't thought of that

Wonder what the forum's policy on that is or should be.

[nctnico]

This is not a homework assignment; I am genuinely trying to grasp these concepts. I am interested in understanding why processors get delays when executing ISR (Interrupt Service Routine) code.  I've heard that it's advisable to keep ISRs short and efficient. Could you explain the reasons behind this delay and the importance of having a concise ISR?"

It seems like homework when you appear to have done literally zero prior work to answer your questions.

I hate to be this guy but:
https://letmegooglethat.com/?q=why+is+desirable+to+keep+ISRs+short

What you are proposing here is the wrong approach. A myth that has many less experienced embedded programmers turn their project into a convoluted mess because they end up chasing rainbows.

[Shonky]

This is not a homework assignment; I am genuinely trying to grasp these concepts. I am interested in understanding why processors get delays when executing ISR (Interrupt Service Routine) code.  I've heard that it's advisable to keep ISRs short and efficient. Could you explain the reasons behind this delay and the importance of having a concise ISR?"

It seems like homework when you appear to have done literally zero prior work to answer your questions.

I hate to be this guy but:
https://letmegooglethat.com/?q=why+is+desirable+to+keep+ISRs+short

What you are proposing here is the wrong approach. A myth that has many less experienced embedded programmers turn their project into a convoluted mess because they end up chasing rainbows.

Don't follow, but when someone comes along looking for an answer served up on a plate without doing even the most basic research for themselves it raises alarms for me.

If they read the first thing they find on the internet and take it as the word and the truth then yeah they'll probably have a bad time but how's that different to asking on a forum?

[dietert1]

Like this.

[Mtech1]

Thank you all for your feedback and concerns. I want to clarify that I'm not a bot, nor am I generating content for AI models or any profit. My intention is to engage in meaningful discussions and gain insights from the expertise of this community.

I understand the importance of conducting research before asking questions, and I'll make an effort to provide more context and share my prior research in future inquiries. I genuinely value the perspectives and knowledge shared here and believe that forums like this provide a valuable platform for learning from diverse experiences.

[SpacedCowboy]

If you have to care about interrupt latency, you are doing something wrong 

Maybe during development, when it’s too late to do anything about it. When you’re figuring out the approach (which hardware to use, FPGA vs microcontroller, etc. etc.)  understanding your constraints is pretty much spot-on IMHO.

Yes. But how very old skool

That would make sense. I am old, and more interested in getting shit done properly than checking boxes on a form. My management actually appreciates that so we get along

[MikeK]

To fully understand interrupt latency, one must study the turbo encabulator and all of it's polymorphic trapulisms.