Is this a thumbs up for memcpy for DUPLICATE_ATOMIC_VECTOR at least ? If there is further specific testing then let me know, happy to help, but you seem to have beaten me to it.
Matthew "Simon Urbanek" <simon.urba...@r-project.org> wrote in message news:65d21b93-a737-4a94-bdf4-ad7e90518...@r-project.org... > > On Apr 21, 2010, at 2:15 PM, Seth Falcon wrote: > >> On 4/21/10 10:45 AM, Simon Urbanek wrote: >>> Won't that miss the last incomplete chunk? (and please don't use >>> DATAPTR on INTSXP even though the effect is currently the same) >>> >>> In general it seems that the it depends on nt whether this is >>> efficient or not since calls to short memcpy are expensive (very >>> small nt that is). >>> >>> I ran some empirical tests to compare memcpy vs for() (x86_64, OS X) >>> and the results were encouraging - depending on the size of the >>> copied block the difference could be quite big: tiny block (ca. n = >>> 32 or less) - for() is faster small block (n ~ 1k) - memcpy is ca. 8x >>> faster as the size increases the gap closes (presumably due to RAM >>> bandwidth limitations) so for n = 512M it is ~30%. >>> >> >>> Of course this is contingent on the implementation of memcpy, >>> compiler, architecture etc. And will only matter if copying is what >>> you do most of the time ... >> >> Copying of vectors is something that I would expect to happen fairly >> often in many applications of R. >> >> Is for() faster on small blocks by enough that one would want to branch >> based on size? >> > > Good question. Given that the branching itself adds overhead possibly not. > In the best case for() can be ~40% faster (for single-digit n) but that > means billions of copies to make a difference (since the operation itself > is so fast). The break-even point on my test machine is n=32 and when I > added the branching it took 20% hit so I guess it's simply not worth it. > The only case that may be worth branching is n:1 since that is likely a > fairly common use (the branching penalty in copy routines is lower than > comparing memcpy/for implementations since the branching can be done > before the outer for loop so this may vary case-by-case). > > Cheers, > Simon > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
