[dpdk-dev] [PATCH] net/i40e: add additional prefetch instructions for bulk rx

[Ferruh Yigit]

On 10/13/2016 11:30 AM, Ananyev, Konstantin wrote:

<...>

>>>>
>>>> Actually I can see some valid use cases where it is beneficial to have this prefetch in driver.
>>>> In our sw distributor case it is trivial to just prefetch next packet on each iteration because packets are processed one by one.
>>>> However when we move this functionality to hw by means of RSS/vfunction/FlowDirector(our long term goal) worker threads will
>> receive
>>>> packets directly from rx queues of NIC.
>>>> First operation of worker thread is to perform bulk lookup in hash table by destination MAC. This will cause cache miss on accessing
>> each
>>>> eth header and can't be easily mitigated in application code.
>>>> I assume it is ubiquitous use case for DPDK.
>>>
>>> Yes it is a quite common use-case.
>>> Though I many cases it is possible to reorder user code to hide (or minimize) that data-access latency.
>>> From other side there are scenarios where this prefetch is excessive and can cause some drop in performance.
>>> Again, as I know, none of PMDs for Intel devices prefetches packet's data in  simple (single segment) RX mode.
>>> Another thing that some people may argue then - why only one cache line is prefetched,
>>> in some use-cases might need to look at 2-nd one.
>>>
>> There is a build-time config setting for this behaviour for exactly the reasons
>> called out here - in some apps you get a benefit, in others you see a perf
>> hit. The default is "on", which makes sense for most cases, I think.
>> From common_base:
>>
>> CONFIG_RTE_PMD_PACKET_PREFETCH=y$
> 
> Yes, but right now i40e and ixgbe non-scattered RX (both vector and scalar) just ignore that flag.
> Though yes, might be a good thing to make them to obey that flag properly.

Hi Vladyslav,

According Konstantin's comment, what do you think updating patch to do
prefetch within CONFIG_RTE_PMD_PACKET_PREFETCH ifdef?

But since config option is enabled by default, performance concern is
still valid and needs to be investigated.

Thanks,
ferruh