Single process shared variable w/o (without) FIFO between deterministic loop.

[Ajay_MV]

Hi,

Why can't we use single process shared variable without FIFO for communicating between deterministic loop?  In other words, why it's always recommended to use FIFO for communications between deterministic loops?

Thanks,

Ajay.

[crossrulz]

Jitter.  When using non-FIFO communications, you introduce jitter into the system, making your deterministic loop not so deterministic.

[Ajay_MV]

And.. Why that's so?  Why there is such jitter causing variable available?  We could always use variable with FIFO if the other non-FIFO vairable causes jitter, isn't it?  Is there any overheads in using non-FIFO variables when compared to FIFO variables?

Thanks,

Ajay.

[crossrulz · Accepted solution]

It's called resource sharing.  When using a single process shared variable, only one thread can access it at a time.  So what happens is A tries to read, but B is busy writing to it.  So A has to wait, adding jitter.

With the RT FIFO enabled, you don't have this issue.  A can read from the FIFO at the same time that B is adding another element.

[Ajay_MV]

Thanks for the clarification 🙂