Re: A question of DPDK ring buffer

[Olivier MATZ <olivier.matz-pdR9zngts4EAvxtiuMwx3w@public.gmane.org>]

Hi Bob,


> 	do {
> 		prod_head = r->prod.head;
> 		cons_tail = r->cons.tail;
> 		prod_next = prod_head + n;
> 		success = rte_atomic32_cmpset(&r->prod.head, prod_head, prod_next);
> 		
> 		/*
> 		  * Why not enqueue data here? It would be just a couple of pointers assignment, not taking too much time.
> 		  * Then the entire CAS loop contains both pointer adjustment and data enqueue, and the dequeue operation would not have a chance to interfere data producing.
>      		  * The next wait loop can be removed accordingly.
> 		/*		

You cannot enqueue your data here: before writing the objects, you must
first check that the cmpset is succesful. In your example, if the cmpset
fails, it would write the objects pointer in a zone already reserved
by another producer core.

The writing of objects must be done after the "do - while" loop, once
cmpset is succesful. But even with that, you cannot remove the wait
loop (r->prod.tail != prod_head) for the reasons described in my
previous mail.

The ring test in app/test/test_ring.c is quite stressful for the rings
so you can use it to check that your solution is working.


Regards,
Olivier