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

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Jun 15, 2021 at 09:53:33AM +0200, Morten Brørup wrote:
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Thomas Monjalon
> > Sent: Tuesday, 15 June 2021 08.48
> > 
> > 14/06/2021 17:48, Morten Brørup:
> > > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Thomas
> > Monjalon
> > > 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?
> > 
> > What is big enough?
> > Is 640KB enough for RAM? ;)
> 
> Good point!
> 
> I think we agree that:
> - The cost of this library is some added complexity, i.e. working with a dynamically sized array through a library instead of just indexing into a compile time fixed size array.
> - The main benefit of this library is saving some RAM (and still allowing a potentially very high number of ports.)
> 
> My point was: The amount of RAM we are saving is a key parameter for the cost/benefit analysis. And since I don't think the rte_eth_devices[] array uses a significant amount of memory, I was asking for some other array using more memory, where the cost/benefit analysis would come out more advantageous to your proposed parray library.
> 
> > 
> > When dealing with microservices switching, the numbers can increase
> > very fast.
> 
> Yes, I strongly supported increasing the port_id type from 8 to 16 bits for this reason, when it was discussed at the DPDK Userspace a few years ago in Dublin. And with large RTE_MAX_QUEUES_PER_PORT values, the rte_eth_dev structure uses quite a lot of space for the rx/tx callback arrays. But the memory usage of rte_eth_devices[] is still relatively insignificant in a system wide context.
> 
> If main purpose is to optimize the rte_eth_devices[] array, I think there are better alternatives than this library. Bruce and Konstantin already threw a few ideas on the table.
>

Yes, though I think we need to be clear on what problems we are trying to
solve here. A generic resizable array may be a useful library for DPDK in
its own right, but for the ethdev (and other devs) arrays I think my
understanding of the problem is that we want:

* scalability of ethdevs list to large numbers of ports, e.g. 2k
* while not paying a large memory footprint penalty for those apps which
  only need a small number of ports, e.g. 2 or 4.

Is that a fair summary?

/Bruce`