[RFC v2] non-temporal memcpy

[Mattias Rönnblom]

On 2022-07-29 18:05, Stephen Hemminger wrote:
> On Fri, 29 Jul 2022 12:13:52 +0000
> Konstantin Ananyev <konstantin.ananyev at huawei.com> wrote:
> 
>> Sorry, missed that part.
>>
>>>    
>>>> Another question - who will do 'sfence' after the copying?
>>>> Would it be inside memcpy_nt (seems quite costly), or would
>>>> it be another API function for that: memcpy_nt_flush() or so?
>>>
>>> Outside. Only the developer knows when it is required, so it wouldn't make any sense to add the cost inside memcpy_nt().
>>>
>>> I don't think we should add a flush function; it would just be another name for an already existing function. Referring to the required
>>> operation in the memcpy_nt() function documentation should suffice.
>>>    
>>
>> Ok, but again wouldn't it be arch specific?
>> AFAIK for x86 it needs to boil down to sfence, for other architectures - I don't know.
>> If you think there already is some generic one (rte_wmb?) that would always produce
>> correct instructions - sure let's use it.
>>   
>>   
> 
> It makes sense in a few select places to use non-temporal copy.
> But it would add unnecessary complexity to DPDK if every function in DPDK that could
> cause a copy had a non-temporal variant.

A NT load and NT store variant, plus a NT load+store variant. :)

> 
> Maybe just having rte_memcpy have a threshold (config value?) that if copy is larger than
> a certain size, then it would automatically be non-temporal.  Small copies wouldn't matter,
> the optimization is more about not stopping cache size issues with large streams of data.

I don't think there's any way for rte_memcpy() to know if the 
application plan to use the source, the destination, both, or neither of 
the buffers in the immediate future. For huge copies (MBs or more) the 
size heuristic makes sense, but for medium sized copies (say a packet 
worth of data), I'm not so sure.

What is unclear to me is if there is a benefit (or drawback) of using 
the imaginary rte_memcpy_nt(), compared to doing rte_memcpy() + 
clflushopt or cldemote, in the typical use case (if there is such).