Re: [dpdk-dev] [PATCH] parray: introduce internal API for dynamic arrays

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Thomas Monjalon
> Sent: Monday, 14 June 2021 15.32
> 
> 14/06/2021 15:15, Bruce Richardson:
> > On Mon, Jun 14, 2021 at 02:22:42PM +0200, Morten Brørup wrote:
> > > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Thomas
> Monjalon
> > > > Sent: Monday, 14 June 2021 12.59
> > > >
> > > > Performance of access in a fixed-size array is very good
> > > > because of cache locality
> > > > and because there is a single pointer to dereference.
> > > > The only drawback is the lack of flexibility:
> > > > the size of such an array cannot be increase at runtime.
> > > >
> > > > An approach to this problem is to allocate the array at runtime,
> > > > being as efficient as static arrays, but still limited to a
> maximum.
> > > >
> > > > That's why the API rte_parray is introduced,
> > > > allowing to declare an array of pointer which can be resized
> > > > dynamically
> > > > and automatically at runtime while keeping a good read
> performance.
> > > >
> > > > After resize, the previous array is kept until the next resize
> > > > to avoid crashs during a read without any lock.
> > > >
> > > > Each element is a pointer to a memory chunk dynamically
> allocated.
> > > > This is not good for cache locality but it allows to keep the
> same
> > > > memory per element, no matter how the array is resized.
> > > > Cache locality could be improved with mempools.
> > > > The other drawback is having to dereference one more pointer
> > > > to read an element.
> > > >
> > > > There is not much locks, so the API is for internal use only.
> > > > This API may be used to completely remove some compilation-time
> > > > maximums.
> > >
> > > I get the purpose and overall intention of this library.
> > >
> > > I probably already mentioned that I prefer "embedded style
> programming" with fixed size arrays, rather than runtime
> configurability. It's my personal opinion, and the DPDK Tech Board
> clearly prefers reducing the amount of compile time configurability, so
> there is no way for me to stop this progress, and I do not intend to
> oppose to this library. :-)
> > >
> > > This library is likely to become a core library of DPDK, so I think
> it is important getting it right. Could you please mention a few
> examples where you think this internal library should be used, and
> where it should not be used. Then it is easier to discuss if the border
> line between control path and data plane is correct. E.g. this library
> is not intended to be used for dynamically sized packet queues that
> grow and shrink in the fast path.
> > >
> > > If the library becomes a core DPDK library, it should probably be
> public instead of internal. E.g. if the library is used to make
> RTE_MAX_ETHPORTS dynamic instead of compile time fixed, then some
> applications might also need dynamically sized arrays for their
> application specific per-port runtime data, and this library could
> serve that purpose too.
> > >
> >
> > Thanks Thomas for starting this discussion and Morten for follow-up.
> >
> > My thinking is as follows, and I'm particularly keeping in mind the
> cases
> > of e.g. RTE_MAX_ETHPORTS, as a leading candidate here.
> >
> > While I dislike the hard-coded limits in DPDK, I'm also not convinced
> that
> > we should switch away from the flat arrays or that we need fully
> dynamic
> > arrays that grow/shrink at runtime for ethdevs. I would suggest a
> half-way
> > house here, where we keep the ethdevs as an array, but one
> allocated/sized
> > at runtime rather than statically. This would allow us to have a
> > compile-time default value, but, for use cases that need it, allow
> use of a
> > flag e.g.  "max-ethdevs" to change the size of the parameter given to
> the
> > malloc call for the array.  This max limit could then be provided to
> apps
> > too if they want to match any array sizes. [Alternatively those apps
> could
> > check the provided size and error out if the size has been increased
> beyond
> > what the app is designed to use?]. There would be no extra
> dereferences per
> > rx/tx burst call in this scenario so performance should be the same
> as
> > before (potentially better if array is in hugepage memory, I
> suppose).

If performance can be improved by allocating array memory differently, we can just allocate memory differently - dynamically sized arrays are not required. :-)

> 
> I think we need some benchmarks to decide what is the best tradeoff.

While performance is always important, the DPDK community seems willing to trade in a little bit of performance for obtaining some other great benefit. I agree with this pragmatic approach. However, the word "tradeoff" triggered another line of thinking:

Regarding this library, we must carefully consider if the benefit is worth the added complexity. We shouldn't introduce additional complexity only to save a few MB of memory, and for the pure principle of avoiding compile time configuration parameters.

It would be much simpler to just increase RTE_MAX_ETHPORTS to something big enough to hold a sufficiently large array. And possibly add an rte_max_ethports variable to indicate the number of populated entries in the array, for use when iterating over the array.

Can we come up with another example than RTE_MAX_ETHPORTS where this library provides a better benefit?

> I spent time on this implementation, but sorry I won't have time for
> benchmarks.
> Volunteers?