* [dpdk-dev] rte_port_ring and SP/MP, SC/MC flags
@ 2017-01-30 5:42 Yerden Zhumabekov
2017-02-01 23:00 ` Dumitrescu, Cristian
0 siblings, 1 reply; 2+ messages in thread
From: Yerden Zhumabekov @ 2017-01-30 5:42 UTC (permalink / raw)
To: dev
Hello,
I'd like to use rte_port_ring abstract in my application and I'm a
little confused about how it treats underlying ring flags.
According to DPDK API reference, when creating a ring (via
rte_ring_create()/rte_ring_init()), RING_F_SP_ENQ/RING_F_SC_DEQ may be
specified. These flags affect the choice of MP/SP, MC/SC operation when
using 'default' ring enq/deq API, i.e.
rte_ring_enqueue()/rte_ring_dequeue(),
rte_ring_enqueue_bulk()/rte_ring_dequeue_bulk(),
rte_ring_enqueue_burst()/rte_ring_dequeue_burst().
These API then choose which version of enq/deq to use considering the
flags. If you use designated API straightforward, those API (*_mp_*,
*_sc_* etc.) don't care about these flags and perform required
operations right away.
When I use rte_port_ring abstraction, '.f_create()' functions check for
flags which were used when creating an underlying ring (see
lib/librte_port/rte_port_ring.c:75). But then different call tables use
designated ring API which makes checking flags pointless.
I find it confusing to be forced to choose between SP/MP, SC/MC twice,
when creating ring at first and creating abstraction afterwards. And I
see no point in checking for ring flags when creating abstraction
because it really does not affect the operation of this abstraction
anyway. Is this behaviour anyhow justified?
--
Yerden Zhumabekov
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [dpdk-dev] rte_port_ring and SP/MP, SC/MC flags
2017-01-30 5:42 [dpdk-dev] rte_port_ring and SP/MP, SC/MC flags Yerden Zhumabekov
@ 2017-02-01 23:00 ` Dumitrescu, Cristian
0 siblings, 0 replies; 2+ messages in thread
From: Dumitrescu, Cristian @ 2017-02-01 23:00 UTC (permalink / raw)
To: Yerden Zhumabekov, dev
Hi Yerden,
> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Yerden
> Zhumabekov
> Sent: Monday, January 30, 2017 5:43 AM
> To: dev@dpdk.org
> Subject: [dpdk-dev] rte_port_ring and SP/MP, SC/MC flags
>
> Hello,
>
> I'd like to use rte_port_ring abstract in my application and I'm a
> little confused about how it treats underlying ring flags.
>
> According to DPDK API reference, when creating a ring (via
> rte_ring_create()/rte_ring_init()), RING_F_SP_ENQ/RING_F_SC_DEQ may
> be
> specified. These flags affect the choice of MP/SP, MC/SC operation when
> using 'default' ring enq/deq API, i.e.
> rte_ring_enqueue()/rte_ring_dequeue(),
> rte_ring_enqueue_bulk()/rte_ring_dequeue_bulk(),
> rte_ring_enqueue_burst()/rte_ring_dequeue_burst().
>
> These API then choose which version of enq/deq to use considering the
> flags. If you use designated API straightforward, those API (*_mp_*,
> *_sc_* etc.) don't care about these flags and perform required
> operations right away.
>
> When I use rte_port_ring abstraction, '.f_create()' functions check for
> flags which were used when creating an underlying ring (see
> lib/librte_port/rte_port_ring.c:75). But then different call tables use
> designated ring API which makes checking flags pointless.
>
Yes, we can detect the SP/MP and SC/MC characteristic of each rte_ring by looking inside the structure, at least at this point. As a side note, personally I see this as an implementation detail that might change, and I don't want to rely on it too much (yes, we do rely on it for validation purpose at port_ring creation time, as you state).
> I find it confusing to be forced to choose between SP/MP, SC/MC twice,
> when creating ring at first and creating abstraction afterwards. And I
> see no point in checking for ring flags when creating abstraction
> because it really does not affect the operation of this abstraction
> anyway. Is this behaviour anyhow justified?
>
Yes, we could have decided to have a unified implementation of rte_port_ring reader/writer that handles the SC/MC or SP/MP aspect transparently, but we decided against it because we want to write branchless code. The unified implementation would have to test the internal S/M flag on every call of the port_ring RX/TX function, which would have a performance impact, especially when a mix of S and M port rings are used within the same app.
>
> --
>
> Yerden Zhumabekov
Regards,
Cristian
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