Is the maximum interrupt frequency for the linux kernel in Hz, kHz, MHz, or GHz?

Question

Before I start: yes, I'm aware that the answer is architecture dependent - I'm just interested in a ballpark figure, in terms of orders of magnitude.

Is there an upper limit imposed by the linux kernel on interrupt frequency?

Background: I want to interface with a camera module from within Linux. The module has a clocked parallel data output (8 bits, at ~650kHz), which I want to read data from and store in a buffer for access through, eg, /dev/camera.

I have a basic driver written, and it is monitoring the appropriate interrupt line. If I leave a wire hanging off the interrupt pin, I get interrupts from white noise. However, if I hook up a higher frequency signal (atm ~250kHz from a 555 timer) then no interrupts are triggered. (I've confirmed this with /proc/interrupts)

My thinking is that this can either be from the GPIO module on the processor not being able to deal with such high frequencies (which would be silly - that's not particularly high), or it could be a kernel issue. What do people think?

Answer 1

Look at it this way. Modern CPUs execute around 10⁹ instructions per second.

In order to handle an interrupt you need to execute some 100-1000 instructions (save the context, do I/O, signal end of interrupt handling, restore the context). That gives you some 10⁶ - 10⁷ interrupts per second max.

If you spend all the time in handling interrupts, then nothing is left for the rest of the system and programs.

So, think of some 10⁵ interrupts/second (100 KHz) being the maximum practical interrupt rate.

There may be other limitations imposed by the circuitry and I'm not too familiar with this aspect. But it's unlikely for the kernel to somehow explicitly limit the interrupt rate. I see no good reason for it and I don't think it's something that can be easily done either.

Now, there are things like DMA that let you have interrupts not on every byte of input/output data, but on a buffer of several kilobytes or even megabytes. E.g. you prepare your data for output in a memory buffer and tell to the DMA controller that it can now send it out from the buffer. When done, it will trigger an interrupt signalling the completion of the transfer and you'll be able to initiate another one. It works the same in the other direction of transfers. You get an interrupt when the entire buffer is filled with input data.

I think you may be facing a hardware limitation if you can receive interrupts at lower rates only.