On May 05 2011, at 16:46, David Gilbert was caught saying: > On 5 May 2011 16:08, Måns Rullgård <m...@mansr.com> wrote: > > David Gilbert <david.gilb...@linaro.org> writes: > >> Not quite: > >> a) Neon memcpy/memset is worse on A9 than non-neon versions (better > >> on A8 typically) > > > > That is not my experience at all. On the contrary, I've seen memcpy > > throughput on A9 roughly double with use of NEON for large copies. > > For small copies, plain ARM is might be faster since the overhead of > > preparing for a properly aligned NEON loop is avoided. > > > > What do you base your claims on? > > My tests here: > https://wiki.linaro.org/WorkingGroups/ToolChain/Benchmarks/InitialMemcpy > > at the bottom of the page are sets of graphs for A9 (left) and A8 (right); > on A9 the Neon memcpy's (red and green) top out much lower than their non-neon > best equivalents (black and cyan). I've seen different results for > very non-aligned > copies, where the vld/vst on Neon work very well.
Looking at the top part of the page, it looks like when doing large size copies, NEON has an obvious advantage; however, I'm not sure how often we do copies of that magnitude in the kernel (I would hope rarely) but I don't know that we have numbers tracking average copy sizes for different workloads. I don't think going for a one-size-fits all approach is the ideal and instead we should provide both build and and runtime configurability (something similar to the RAID code's boot-up performance tests) to allow for selection of the appropriate memcpy implementation. > > I don't see the connection between Thumb2 and memcpy performance. > > Thumb2 can do anything 32-bit ARM can. > > There are the purists who says write everything in Thumb2 now; however > there is an > interesting question of which is faster, and IMHO the ARM code is > likely to be a bit > faster in most cases. Do we have numbers for this? :) ~Deepak -- "People think all we need to fix our predicament is a free source of energy, but I think we need to change out behaviour. More energy would just deplete the Earth's lifeblood faster." - Janine Benyius _______________________________________________ linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
