On Tue, May 28, 2024 at 10:19:15AM +0200, Mattias Rönnblom wrote: > On 2024-05-28 09:43, Mattias Rönnblom wrote: > > Provide build option to have functions in <rte_memcpy.h> delegate to > > the standard compiler/libc memcpy(), instead of using the various > > traditional, handcrafted, per-architecture rte_memcpy() > > implementations. > > > > A new meson build option 'use_cc_memcpy' is added. The default is > > true. It's not obvious what should be the default, but compiler > > memcpy() is enabled by default in this RFC so any tests run with this > > patch use the new approach. > > > > One purpose of this RFC is to make it easy to evaluate the costs and > > benefits of a switch. > > > > I've tested this patch some with DSW micro benchmarks, and the result is a > 2.5% reduction of the DSW+testapp overhead with cc/libc memcpy. GCC 11.4. > > We've also run characteristic test suite of a large, real world app. Here, > we saw no effect. GCC 10.5. > > x86_64 in both cases (Skylake and Raptor Lake). > > Last time we did the same, there were a noticeable performance degradation > in both the above cases. > > This is not a lot of data points, but I think it we should consider making > the custom RTE memcpy() implementations optional in the next release, and if > no-one complains, remove the implementations in the next release. > > (Whether or not [or how long] to keep the wrapper API is another question.) > > <snip>
The other instance I've heard mention of in the past is virtio/vhost, which used to have a speedup from the custom memcpy. My own thinking on these cases, is that for targetted settings like these, we should look to have local memcpy functions written - taking account of the specifics of each usecase. For virtio/vhost for example, we can have assumptions around host buffer alignment, and we also can be pretty confident we are copying to another CPU. For DSW, or other eventdev cases, we would only be looking at copies of multiples of 16, with guaranteed 8-byte alignment on both source and destination. Writing efficient copy fns for specific scenarios can be faster and more effective than trying to write a general, optimized in all cases, memcpy. It also discourages the use of non-libc memcpy except where really necessary. Naturally, if we find there are a lot of cases where use of libc memcpy slows us down, we will want to keep a general rte_memcpy. However, I'd hope the slowdown cases are very few. /Bruce
