Re: [RFC 1/2] config: add optimal burst size configuration

[Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>]

>> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>
>> 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 <pbhagavatula@marvell.com>
>> ---
>> 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)
>

Agreed, would the 

><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)