[RFC] Define well known packet burst sizes

[RFC] Define well known packet burst sizes

[Morten Brørup]

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 *))

RE: [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 *))

RE: [RFC] Define well known packet burst sizes

[Morten Brørup]

> From: Pavan Nikhilesh Bhagavatula [mailto:pbhagavatula@marvell.com]
> Sent: Saturday, 12 October 2024 15.55
> 
> > 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

Agree. The default burst size should be platform specific for optimal performance. The "generic" platform value can be 32.

And applications with different needs can use a meson option to override the platform specific default.

The other two suggested "well known" (intended to be commonly used) burst sizes can reside 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 *))