[RFC] Define well known packet burst sizes

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

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

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 *))