[dpdk-dev] rte_ring features in use (or not)

Wiles, Keith keith.wiles at intel.com
Wed Jan 25 23:27:31 CET 2017

> On Jan 25, 2017, at 9:57 AM, Richardson, Bruce <bruce.richardson at intel.com> wrote:
> On Wed, Jan 25, 2017 at 03:59:55PM +0000, Wiles, Keith wrote:
>> Sent from my iPhone
>>> On Jan 25, 2017, at 7:48 AM, Bruce Richardson <bruce.richardson at intel.com> wrote:
>>>> On Wed, Jan 25, 2017 at 01:54:04PM +0000, Bruce Richardson wrote:
>>>>> On Wed, Jan 25, 2017 at 02:20:52PM +0100, Olivier MATZ wrote:
>>>>> On Wed, 25 Jan 2017 12:14:56 +0000, Bruce Richardson
>>>>> <bruce.richardson at intel.com> wrote:
>>>>>> Hi all,
>>>>>> while looking at the rte_ring code, I'm wondering if we can simplify
>>>>>> that a bit by removing some of the code it in that may not be used.
>>>>>> Specifically:
>>>>>> * Does anyone use the NIC stats functionality for debugging? I've
>>>>>> certainly never seen it used, and it's presence makes the rest less
>>>>>> readable. Can it be dropped?
>>>>> What do you call NIC stats? The stats that are enabled with
>>>> Yes. By NIC I meant ring. :-(
>>> <snip>
>>>>> For the ring, in my opinion, the stats could be fully removed.
>>>> That is my thinking too. For mempool, I'd wait to see the potential
>>>> performance hits before deciding whether or not to enable by default.
>>>> Having them run-time enabled may also be an option too - if the branches
>>>> get predicted properly, there should be little to no impact as we avoid
>>>> all the writes to the stats, which is likely to be where the biggest hit
>>>> is.
>>>>>> * RTE_RING_PAUSE_REP_COUNT is set to be disabled at build time, and
>>>>>> so does anyone actually use this? Can it be dropped?
>>>>> This option looks like a hack to use the ring in conditions where it
>>>>> should no be used (preemptable threads). And having a compile-time
>>>>> option for this kind of stuff is not in vogue ;)
>>> <snip>
>>>>>> * Who uses the watermarks feature as is? I know we have a sample app
>>>>>> that uses it, but there are better ways I think to achieve the same
>>>>>> goal while simplifying the ring implementation. Rather than have a
>>>>>> set watermark on enqueue, have both enqueue and dequeue functions
>>>>>> return the number of free or used slots available in the ring (in
>>>>>> case of enqueue, how many free there are, in case of dequeue, how
>>>>>> many items are available). Easier to implement and far more useful to
>>>>>> the app.
>>>>> +1
>>> Bonus question:
>>> * Do we know how widely used the enq_bulk/deq_bulk functions are? They
>>> are useful for unit tests, so they do have uses, but I think it would
>>> be good if we harmonized the return values between bulk and burst
>>> functions. Right now:
>>>   enq_bulk  - only enqueues all elements or none. Returns 0 for all, or
>>>               negative error for none.
>>>   enq_burst - enqueues as many elements as possible. Returns the number
>>>               enqueued.
>> I do use the apis in pktgen and the difference in return values has got me once. Making them common would be great,  but the problem is backward compat to old versions I would need to have an ifdef in pktgen now. So it seems like we moved the problem to the application.
> Yes, an ifdef would be needed, but how many versions of DPDK back do you
> support? Could the ifdef be removed again after say, 6 months?

I have people trying to run 2.1 and 2.2 versions of Pktgen. I can cut them off, but I would prefer not to.
>> I would like to see the old API kept and a new API with the new behavior. I know it adds another API but one of the API would be nothing more than wrapper function if not a macro. 
>> Would that be more reasonable then changing the ABI?
> Technically, this would be an API rather than ABI change, since the
> functions are inlined in the code. However, it's not the only API change
> I'm looking to make here - I'd like to have all the functions start
> returning details of the state of the ring, rather than have the
> watermarks facility. If we add all new functions for this and keep the
> old ones around, we are just increasing our maintenance burden.
> I'd like other opinions here. Do we see increasing the API surface as
> the best solution, or are we ok to change the APIs of a key library like
> the rings one?
> /Bruce


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