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

["Van Haaren, Harry" <harry.van.haaren@intel.com>]

> -----Original Message-----
> From: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> Sent: Wednesday, October 27, 2021 12:42 PM
> To: Van Haaren, Harry <harry.van.haaren@intel.com>; Thomas Monjalon
> <thomas@monjalon.net>; Aman Kumar <aman.kumar@vvdntech.in>
> Cc: dev@dpdk.org; viacheslavo@nvidia.com; Burakov, Anatoly
> <anatoly.burakov@intel.com>; Song, Keesang <Keesang.Song@amd.com>;
> jerinjacobk@gmail.com; Ananyev, Konstantin <konstantin.ananyev@intel.com>;
> Richardson, Bruce <bruce.richardson@intel.com>;
> honnappa.nagarahalli@arm.com; Ruifeng Wang <ruifeng.wang@arm.com>;
> David Christensen <drc@linux.vnet.ibm.com>; david.marchand@redhat.com;
> stephen@networkplumber.org
> Subject: Re: [dpdk-dev] [PATCH v4 2/2] lib/eal: add temporal store memcpy
> support for AMD platform
> 
> On 2021-10-27 13:03, Van Haaren, Harry wrote:
> >> -----Original Message-----

<snip>

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.

-Harry

<snip remaining points>