[RFC PATCH] eventdev: ensure 16-byte alignment for events

[Mattias Rönnblom]

On 2023-10-06 14:19, Bruce Richardson wrote:
> On Fri, Oct 06, 2023 at 02:15:00PM +0200, Mattias Rönnblom wrote:
>> On 2023-10-05 13:51, Bruce Richardson wrote:
>>> The event structure in DPDK is 16-bytes in size, and events are
>>> regularly passed as parameters directly rather than being passed as
>>> pointers.
>>
>> When are events passed by-value, rather than by-reference? There are no such
>> examples in the public eventdev API.
>>
>> To help compiler optimize correctly, we can explicitly request
>>> 16-byte alignment for events, which means that we should be able
>>> to do aligned vector loads/stores (e.g. with SSE or Neon) when working
>>> with those events.
>>>
>>
>> That change is both helping and sabotaging the optimizer's work. Now every
>> stack allocation needs to be 2-byte aligned - in DPDK code, and in the
>> application. >>
>> The effect this change has on an eventdev app using DSW is a ~3 cycle/event
>> performance degradation on an AMD Zen 3 system, and a ~4 cycle/event
>> performance degradation on a Skylake-generation Intel CPU.
>>
> 
> Thanks for checking - this is the sort of feedback needed alright. In SW
> eventdev we copy events around alot without using pointers, so I felt that
> alignment would be helpful to avoid issues with events spanning cachelines.
> 
> However, since it has negative impacts, I'm quite happy to drop the idea,
> and keep things as they are. I'll mark the change as rejected in patchwork.
> 

I think this was an excellent idea, it just didn't happen to have the 
desired effect. At least not in the particular cases where I evaluated it.

Given the complexity of compilers and CPUs, it's almost impossible to 
predict the outcome, performance-wise, of a particular change.