Re: [SPDK] SPDK Ring Enqueue/Dequeue Issue

[Harris, James R <james.r.harris at intel.com>]

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On 9/19/18, 11:37 AM, "SPDK on behalf of Andrey Kuzmin" <spdk-bounces(a)lists.01.org on behalf of andrey.v.kuzmin(a)gmail.com> wrote:

    On Wed, Sep 19, 2018 at 9:13 PM John Barnard <john.barnard(a)broadcom.com>
    wrote:
    
<snip>

    >
    > It seems that the spdk_ring_enqueue function calls
    > rte_ring_mp_enqueue_bulk(), while spdk_ring_dequeue function calls
    > rte_ring_sc_dequeue_burst().  When we change it to call
    > ret_ring_mp_dequeue_burst(), the problem went away.  So my question is, why
    > this difference in the rte calls made by the SPDK?  Is this a bug or on
    > purpose?  Also, we noticed that there is no ring create flag (in
    > include/spdk/env.h) for multi-producer, multi-consumer (i.e.
    > SPDK_RING_TYPE_MP_MC), although it doesn't seem to matter if we pass the
    > SPDK_RING_TYPE_MP_SC flag to  spdk_ring_create() (i.e. it's the call to
    > rte_ring_mp_dequeue_burst that's critical).  Is there a reason this flag
    > was left out?
    >
    
    The two issues above are interrelated. SPDK seemingly does not care for
    MP/MC rings, so it neither defines the flag to create the respective ring
    type, nor appreciates this ring type under ring_dequeue (for enqueue, it
    uses MP flavor which works for SP case as well).
    
    The definitive answer as to why belongs to the SPDK team, while my guess is
    that rings in SPDK are primarily (and solely, to the best of my knowledge)
    for passing messages between SPDK threads where  MP/MC scenario does not
  apply.

Andrey is correct.  There hasn’t been a use case for MP/MC rings in SPDK yet.  Rings are primarily used today for passing messages to a specific reactor – each reactor has its own ring (so it’s single consumer) but any thread may post messages to it.

I see no reason to not add MP/MC support if it’s needed for NVMe-oF FC.

John – is this something you’d like to tackle?

-Jim