[RFC] Define well known packet burst sizes

[Pavan Nikhilesh Bhagavatula]

> We should define some "well known" packet burst sizes in rte_config.h.
> 

A while back we had the same idea, except that it should be platform specific
Ex., for CNXK optimal burst size across workloads is 64.

Instead of rte_config.h maybe we should have it as a meson option in 
meson_options.txt with default as 32.

That way platforms can also modify it in config/<arch>/meson.build

> Especially the default packet burst size is interesting;
> if known at compile time, various drivers and libraries can optimize for it (i.e.
> special handling for nb_pkts == RTE_PKT_BURST_DEFAULT).
> 
> It should also be used in DPDK examples and apps, instead of defining
> MAX_PKT_BURST in each and everyone.
> 
> 
> Specifically:
> 
> /**
>  * Default packet burst size.
>  *
>  * Also intended for optimizing packet processing (e.g. by loop unrolling).
>  */
> #define RTE_PKT_BURST_DEFAULT 32
> 
> /**
>  * Largest packet burst size guaranteed to be supported throughout DPDK.
>  *
>  * Also intended for sizing large temporary arrays of mbufs, e.g. in
> rte_pktmbuf_free_bulk().
>  */
> #define RTE_PKT_BURST_MAX 512
> #define RTE_MEMPOOL_CACHE_MAX_SIZE RTE_PKT_BURST_MAX
> 
> /**
>  * Smallest packet burst size recommended for latency sensitive applications,
> when throughput still matters.
>  *
>  * Also intended for sizing small staging arrays of mbufs, e.g. in drivers.
>  *
>  * Note: Corresponds to one CPU cache line of object pointers.
>  *  - 8 on most 64 bit architectures, 16 on POWER architecture (ppc_64).
>  *  - 16 on all 32 bit architectures.
>  */
> #define RTE_PKT_BURST_SMALL (RTE_CACHE_LINE_SIZE / sizeof(void *))