> From: Pavan Nikhilesh <[email protected]>
>
> Add RTE_OPTIMAL_BURST_SIZE to allow platforms to configure the
> optimal burst size.
>
> Set default value to 64 for soc_cn10k and 32 generally.
>
> Signed-off-by: Pavan Nikhilesh <[email protected]>
> ---
> This improves performance by 5% on l2fwd, other examples showed
> negligible difference on CN10K.
>
I support the concept of having a recommended mbuf burst size, targeting the
majority of generic applications.
Making it CPU dependent seems like a good choice.
It should be named differently.
First of all, "optimal" depends on the use case; if targeting low latency,
shorter bursts are better, so "OPTIMAL" should not be part of the name.
Second, I would guess that it only targets mbuf bursts, not also bursts of
other operations (e.g. hash lookups), so "MBUF" should be part of the name.
Suggestion:
/* Recommended burst size for generic applications, striking a balance between
throughput and latency. */
dpdk_conf.set('RTE_MBUF_BURST_SIZE_MAX' (or _DEFAULT), 64)
<feature creep>
/* Recommended burst size for generic applications targeting low latency. */
dpdk_conf.set('RTE_MBUF_BURST_SIZE_MIN', 4)
</feature creep>
Having these standardized will also allow libraries and drivers to optimize for
them, e.g. drivers should support bursts sizes all the way down to
RTE_MBUF_BURST_SIZE_MIN, and can static_assert() that the
RTE_MBUF_BURST_SIZE_MIN is not lower than supported by the driver/hardware.
<more feature creep>
rte_config.h could have "#define RTE_MBUF_BURST_SIZE RTE_MBUF_BURST_SIZE_MAX",
for the application developer to change to RTE_MBUF_BURST_SIZE_MIN for low
latency applications.
This will let the libraries and drivers optimize for the specific burst size
used by the application.
</more feature creep>
<rambling>
Intuitively, I would assume that the optimal burst size essentially depends on
the CPU's L1D cache size and the application's number of non-mbuf cache lines
accessed per burst.
Let's say a CPU core has 32 KiB cache (= 512 cache lines), and each burst
touches 4 cache lines per packet:
2 cache lines for the mbuf
1 cache line for the packet data
1 cache line per packet for some table lookup/forwarding entry
Then the mbuf burst should be max 512/4 = 128.
But local variables also use memory during processing, so using a burst of 64
would leave room for that and some more.
</rambling>
> config/arm/meson.build | 1 +
> config/meson.build | 1 +
> 2 files changed, 2 insertions(+)
>
> diff --git a/config/arm/meson.build b/config/arm/meson.build
> index 523b0fc0ed50..fa64c07016b1 100644
> --- a/config/arm/meson.build
> +++ b/config/arm/meson.build
> @@ -481,6 +481,7 @@ soc_cn10k = {
> ['RTE_MAX_LCORE', 24],
> ['RTE_MAX_NUMA_NODES', 1],
> ['RTE_MEMPOOL_ALIGN', 128],
> + ['RTE_OPTIMAL_BURST_SIZE', 64],
> ],
> 'part_number': '0xd49',
> 'extra_march_features': ['crypto'],
> diff --git a/config/meson.build b/config/meson.build
> index 0cb074ab95b7..95367ae88e2d 100644
> --- a/config/meson.build
> +++ b/config/meson.build
> @@ -386,6 +386,7 @@ if get_option('mbuf_refcnt_atomic')
> dpdk_conf.set('RTE_MBUF_REFCNT_ATOMIC', true)
> endif
> dpdk_conf.set10('RTE_IOVA_IN_MBUF', get_option('enable_iova_as_pa'))
> +dpdk_conf.set('RTE_OPTIMAL_BURST_SIZE', 32)
>
> compile_time_cpuflags = []
> subdir(arch_subdir)
> --
> 2.50.1 (Apple Git-155)
