Re: [PATCH] ixgbe: prefetch packet headers in vector PMD receive function

[Zoltan Kiss <zoltan.kiss@linaro.org>]

On 07/09/15 07:41, Richardson, Bruce wrote:
>
>> -----Original Message-----
>> From: Zoltan Kiss [mailto:zoltan.kiss@linaro.org]
>> Sent: Monday, September 7, 2015 3:15 PM
>> To: Richardson, Bruce; dev@dpdk.org
>> Cc: Ananyev, Konstantin
>> Subject: Re: [PATCH] ixgbe: prefetch packet headers in vector PMD receive
>> function
>>
>>
>>
>> On 07/09/15 13:57, Richardson, Bruce wrote:
>>>
>>>> -----Original Message-----
>>>> From: Zoltan Kiss [mailto:zoltan.kiss@linaro.org]
>>>> Sent: Monday, September 7, 2015 1:26 PM
>>>> To: dev@dpdk.org
>>>> Cc: Ananyev, Konstantin; Richardson, Bruce
>>>> Subject: Re: [PATCH] ixgbe: prefetch packet headers in vector PMD
>>>> receive function
>>>>
>>>> Hi,
>>>>
>>>> I just realized I've missed the "[PATCH]" tag from the subject. Did
>>>> anyone had time to review this?
>>>>
>>> Hi Zoltan,
>>>
>>> the big thing that concerns me with this is the addition of new
>>> instructions for each packet in the fast path. Ideally, this
>>> prefetching would be better handled in the application itself, as for
>>> some apps, e.g. those using pipelining, the core doing the RX from the
>>> NIC may not touch the packet data at all, and the prefetches will
>> instead cause a performance slowdown.
>>> Is it possible to get the same performance increase - or something
>>> close to it - by making changes in OVS?
>> OVS already does a prefetch when it's processing the previous packet, but
>> apparently it's not early enough. At least for my test scenario, where I'm
>> forwarding UDP packets with the least possible overhead. I guess in tests
>> where OVS does more complex processing it should be fine.
>> I'll try to move the prefetch earlier in OVS codebase, but I'm not sure if
>> it'll help.
> I would suggest trying to prefetch more than one packet ahead. Prefetching 4 or
> 8 ahead might work better, depending on the processing being done.

I've moved the prefetch earlier, and it seems to work:

https://patchwork.ozlabs.org/patch/519017/

However it raises the question: should we remove header prefetch from 
all the other drivers, and the CONFIG_RTE_PMD_PACKET_PREFETCH option?

>
> /Bruce