Re: [PATCH v6 4/4] hash: add SVE support for bulk key lookup

[Yoan Picchi <yoan.picchi@foss.arm.com>]

On 3/12/24 03:57, fengchengwen wrote:
> Hi Yoan,
> 
> On 2024/3/12 7:21, Yoan Picchi wrote:
>> - Implemented SVE code for comparing signatures in bulk lookup.
>> - Added Defines in code for SVE code support.
>> - Optimise NEON code
> 
> This commit does not include this part. Pls only describe the content in this commit.

Thank you. I forgot to edit that out after moving commit around.

> 
>> - New SVE code is ~5% slower than optimized NEON for N2 processor.
>>
>> Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
>> Signed-off-by: Harjot Singh <harjot.singh@arm.com>
>> Reviewed-by: Nathan Brown <nathan.brown@arm.com>
>> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
>> ---
>>   lib/hash/arch/arm/compare_signatures.h | 58 ++++++++++++++++++++++++++
>>   lib/hash/rte_cuckoo_hash.c             |  2 +
>>   2 files changed, 60 insertions(+)
>>
>> diff --git a/lib/hash/arch/arm/compare_signatures.h b/lib/hash/arch/arm/compare_signatures.h
>> index b5a457f936..8a0627e119 100644
>> --- a/lib/hash/arch/arm/compare_signatures.h
>> +++ b/lib/hash/arch/arm/compare_signatures.h
>> @@ -47,6 +47,64 @@ compare_signatures_dense(uint16_t *hitmask_buffer,
>>   		*hitmask_buffer = vaddvq_u16(hit2);
>>   		}
>>   		break;
>> +#endif
>> +#if defined(RTE_HAS_SVE_ACLE)
>> +	case RTE_HASH_COMPARE_SVE: {
> 
> ...
> 
>>   #endif
>>   	default:
>>   		for (unsigned int i = 0; i < RTE_HASH_BUCKET_ENTRIES; i++) {
>> diff --git a/lib/hash/rte_cuckoo_hash.c b/lib/hash/rte_cuckoo_hash.c
>> index e41f03270a..7a474267f0 100644
>> --- a/lib/hash/rte_cuckoo_hash.c
>> +++ b/lib/hash/rte_cuckoo_hash.c
>> @@ -452,6 +452,8 @@ rte_hash_create(const struct rte_hash_parameters *params)
>>   #elif defined(RTE_ARCH_ARM64)
>>   	if (rte_cpu_get_flag_enabled(RTE_CPUFLAG_NEON)) {
>>   		h->sig_cmp_fn = RTE_HASH_COMPARE_NEON;
>> +		if (rte_cpu_get_flag_enabled(RTE_CPUFLAG_SVE))
>> +			h->sig_cmp_fn = RTE_HASH_COMPARE_SVE;
> 
> The RTE_HASH_COMPARE_SVE was defined in "PATCH v6 1/4] hash: pack the hitmask for hash in bulk lookup",
> but its first use is in this commit, so I think it should defined in this commit.
> 
> If RTE_CPUFLAG_SVE and RTE_HAS_SVE_ACLE both set, then SVE impl will be chosen.
> If RTE_CPUFLAG_SVE defined, but RTE_HAS_SVE_ACLE was not, then scalar will be chosen. --- in this case we could back to NEON impl.
> So I suggest direct use "#if defined(RTE_HAS_SVE_ACLE)" here.

Sounds fair. I'll do it.

> 
>>   	}
>>   	else
>>   #endif
>>
> 
> Plus:
> I notice the commit log said the SVE performance is slower than NEON.
> 
> And I also notice other platform SVE also lower than NEON,
> 1. b4ee9c07bd config/arm: disable SVE ACLE for CN10K
> 2. 4eea7c6461 config/arm: add SVE ACLE control flag
> 
> So maybe we should disable RTE_HAS_SVE_ACLE default by:
> diff --git a/config/arm/meson.build b/config/arm/meson.build
> index 9d6fb87d7f..a5b890d100 100644
> --- a/config/arm/meson.build
> +++ b/config/arm/meson.build
> @@ -875,7 +875,7 @@ endif
> 
>   if cc.get_define('__ARM_FEATURE_SVE', args: machine_args) != ''
>       compile_time_cpuflags += ['RTE_CPUFLAG_SVE']
> -    if (cc.check_header('arm_sve.h') and soc_config.get('sve_acle', true))
> +    if (cc.check_header('arm_sve.h') and soc_config.get('sve_acle', false))
>           dpdk_conf.set('RTE_HAS_SVE_ACLE', 1)
>       endif
>   endif
> 
> If the platform verify SVE has higher performance, then it could enable SVE by add "sve_acle: true" in soc_xxx config.
> 
> Thanks

Here I kinda disagree. In this particular instance, SVE is a bit slower 
with narrow vectors (128b), but could be faster with some wider vector 
sizes.
Even in general SVE 128b is not just slower than neon. It's a case by 
case basis. Sometime it's slower, sometime it's faster, so I don't think 
we should just disable it by default. In any case, disabling it should 
be its own patch with much discussion, not just a offhand thing we 
include in the middle of this patch.
This SVE version is still faster than the upstream neon version. I just 
happen to have improved the neon version even more.