Re: [dpdk-dev] [PATCH v7 0/6] Add lock-free ring and mempool handler

["Eads, Gage" <gage.eads@intel.com>]

> 
> On Mon, 18 Mar 2019 21:49:44 +0000
> "Eads, Gage" <gage.eads@intel.com> wrote:
> 
> > Hi all,
> >
> > Friendly reminder that in order to get this feature into 19.08 (assuming
> folks also want that :)), the API deprecation notice needs to be merged into
> 19.05.
> >
> > Thanks,
> > Gage
> 
> Given the recent API/ABI stability discussion, this is the kind of patch that
> really needs to be examined and justified.

Can you point me to the discussion (assuming it was on the ML)? I'm aware of Ferruh's changes to the docs, but not the discussion you referenced.

The lock-free ring functionality itself is a valuable addition to DPDK, primarily because it lacks the standard ring's non-preemptive constraint. The non-preemptive constraint is important in an application with both high priority, performance-sensitive data-plane components and low-priority control-plane components. This was important enough to warrant further clarification recently[1], and has been a discussion topic for some time[2][3].

The modified API, rte_ring_get_memsize(), was added to allow users to initialize rings in separately allocated memory. This function isn't called in DPDK's examples/apps/drivers, and a quick google search didn't turn up any open source projects that call the function, so I suspect that a majority of ring code uses rte_ring_create() instead of rte_ring_get_memsize() + rte_ring_init(). So I suspect this interface change will affect a small percentage of DPDK users.

As a straw-man counter-proposal, we could instead introduce a lock-free specific function rte_lf_ring_get_memsize() that lock-free ring users would call instead of rte_ring_get_memsize(). This would avoid the API modification, but:
- It's awkward to have one rte_lf_ring_* function and otherwise access the lock-free ring through rte_ring_* functions.
- It's also easy to envision a user incorrectly calling rte_ring_get_memsize() rather than rte_lf_ring_get_memsize() for a lock-free ring, since otherwise rte_ring_* functions are used. DPDK would have no way to detect that the allocated memory is too small, and if such a bug occurs it would manifest itself as memory corruption.
- Changing rte_ring_get_memsize() to take a flags argument may be the better long-term design, if another flag is introduced that likewise uses a different ring size.

Another approach is to break out the lock-free ring into a fully separate API. One of the goals of my patchset was to allow applications to switch to lock-free rings with minimal code change; I think the value of the lock-free ring warrants such an approach.

(Unfortunately without hard numbers on the cost or benefit of such a change, these arguments are fairly subjective.)

Thanks,
Gage

[1] https://patches.dpdk.org/patch/43122/
[2] http://mails.dpdk.org/archives/dev/2013-November/000714.html
[3] http://mails.dpdk.org/archives/dev/2014-January/001163.html