[dpdk-dev] [PATCH v4 2/2] lib/eal: add temporal store memcpy support for AMD platform

[Ananyev, Konstantin]

From: Aman Kumar <aman.kumar at vvdntech.in> 
Sent: Wednesday, October 27, 2021 2:35 PM
To: Ananyev, Konstantin <konstantin.ananyev at intel.com>
Cc: Van Haaren, Harry <harry.van.haaren at intel.com>; mattias.ronnblom <mattias.ronnblom at ericsson.com>; Thomas Monjalon <thomas at monjalon.net>; dev at dpdk.org; viacheslavo at nvidia.com; Burakov, Anatoly <anatoly.burakov at intel.com>; Song, Keesang <Keesang.Song at amd.com>; jerinjacobk at gmail.com; Richardson, Bruce <bruce.richardson at intel.com>; honnappa.nagarahalli at arm.com; Ruifeng Wang <ruifeng.wang at arm.com>; David Christensen <drc at linux.vnet.ibm.com>; david.marchand at redhat.com; stephen at networkplumber.org
Subject: Re: [dpdk-dev] [PATCH v4 2/2] lib/eal: add temporal store memcpy support for AMD platform

>> 
>> Hi Mattias,
>> 
>> > > 6) What is the use-case for this? When would a user *want* to use this instead
>> > of rte_memcpy()?
>> > > If the data being loaded is relevant to datapath/packets, presumably other
>> > packets might require the
>> > > loaded data, so temporal (normal) loads should be used to cache the source
>> > data?
>> >
>> >
>> > I'm not sure if your first question is rhetorical or not, but a memcpy()
>> > in a NT variant is certainly useful. One use case for a memcpy() with
>> > temporal loads and non-temporal stores is if you need to archive packet
>> > payload for (distant, potential) future use, and want to avoid causing
>> > unnecessary LLC evictions while doing so.
>> 
>> Yes I agree that there are certainly benefits in using cache-locality hints.
>> There is an open question around if the src or dst or both are non-temporal.
>> 
>> In the implementation of this patch, the NT/T type of store is reversed from your use-case:
>> 1) Loads are NT (so loaded data is not cached for future packets)
>> 2) Stores are T (so copied/dst data is now resident in L1/L2)
>> 
>> In theory there might even be valid uses for this type of memcpy where loaded
>> data is not needed again soon and stored data is referenced again soon,
>> although I cannot think of any here while typing this mail..
>> 
>> I think some use-case examples, and clear documentation on when/how to choose
>> between rte_memcpy() or any (potential future) rte_memcpy_nt() variants is required
>> to progress this patch.
>> 
>> Assuming a strong use-case exists, and it can be clearly indicators to users of DPDK APIs which
>> rte_memcpy() to use, we can look at technical details around enabling the implementation.
>> 
>
> +1 here.
> Function behaviour and restrictions (src parameter needs to be 16/32 B aligned, etc.),
> along with expected usage scenarios have to be documented properly.
> Again, as Harry pointed out, I don't see any AMD specific instructions in this function,
> so presumably such function can go into __AVX2__ code block and no new defines will
> be required. 
> Agreed that APIs are generic but we've kept under an AMD flag for a simple reason that it is NOT tested on any other platform.
> A use-case on how to use this was planned earlier for mlx5 pmd but dropped in this version of patch as the data path of mlx5 is going to be refactored soon and may not be useful for > future versions of mlx5 (>22.02). 
> Ref link: https://patchwork.dpdk.org/project/dpdk/patch/20211019104724.19416-2-aman.kumar@vvdntech.in/(we've plan to adapt this into future version)
> The patch in the link basically enhances mlx5 mprq implementation for our specific use-case and with 128B packet size, we achieve ~60% better perf. We understand the use of this
> copy function should be documented which we shall plan along with few other platform specific optimizations in future versions of DPDK. As this does not conflict with other  >platforms, can we still keep under AMD flag for now as suggested by Thomas?

From what I read above the patch is sort of in the half-ready stage.
Why to rush here and try to push to the DPDK things that doesn't full-fill DPDK policy?
Probably better to do all missing parts first (docs, tests, etc.) and then come-up with updated version.