[ag-automation] Adding OSADL patch to 2.6.33.7 increases latency and CPU usage?

[Carsten Emde]

Grant,

>> The solution is user-space real-time programming using portable and
>> standardized POSIX calls. This is what PREEMPT_RT is made for. Please
>> repeat your tests in user space with and without PREEMPT_RT
> I can't repeat my tests in user space.  I'm testing how long it takes
> a device driver ISR in the kernel to run.  I've got a piece of
> hardware connected to the IRQ0 pin that needs to be serviced by its
> device driver within X microseconds of an interrupt that it produces
> by asserting the signal on the IRQ0 pin.
If X is, in fact, a single-digit number, then you probably want to use 
an FPGA or similar for your project. A high-performance state-of the art 
CPU is no longer able to react that fast. Most of the performance gains 
(and more importantly power-efficiency gains) have been achieved at the 
expense of an increased worst-case latency.

> Ah. So you're saying that the RT patches are to improve _user_space_
> response -- even at the cost of significantly increased CPU usage and
> interrupt latency.
Yes, indeed. However, the concept of doing real-time work in the ISR is 
pretty much related to old-school CPUs such as Motorola's 68xxx and 
similar. They have prioritized interrupts and, more importantly, 
vectorized interrupts with a vector base register. This ensures that the 
CPU can dereference to the ISR within a couple of instructions. Modern 
CPUs and chipsets often do not provide control of interrupt priorities 
and - the greatest invention on earth - shared interrupt lines make 
everything worse. If the real-time interrupt arrives very shortly after 
another interrupt that cannot be given a lower priority and that 
requires lengthy register accesses and data transfers with disabled 
interrupts, you are out of luck.

> Yes, thanks.  If I understand correctly: if low interrupt latency is
> what I care about, then I'm better off without the realtime patch.
Hm, not necessarily - see above.

	-Carsten.