Thanks! In terms of absolute performance, I think it's just that I'm on a slower/older machine with a Core Duo @1.8Ghz ca. 2006. Trying it again on a Xeon server gives 11.8ms and a friend's workstation gives 8.4ms; much closer to the speed you're getting.
Matthew Flatt <mfl...@cs.utah.edu> writes: > These changes look good, and I'll push them. > > I also tried a larger revision to lift the mode tests out of the loop; > it's only worth about 10%, but it might set up further improvements. > > When I run your example, though, I get much better absolute performance > than you're reporting: 34.3 ± 0.2 msec in version 5.3.1, and 8.45 ± 0.1 > msec after the changes. That's running `racket' from a command line and > on 64-bit Mac OS X, but I get similar results from other machines (and > other OSes under VirtualBox on Mac OS X). Any idea why your numbers are > so different? > > At Sun, 16 Dec 2012 11:29:58 -0700, Michael Wilber wrote: >> TL;DR: About ~2.8x speedup from using local variables and unsafe >> functions. Copying each bitmap row could bring speedup to ~20x, but it >> doesn't quite work and I need your help. Pull request at >> https://github.com/plt/racket/pull/199 >> >> Hey there! >> >> I'm writing some FFmpeg bindings for Racket. It's fast enough to decode >> video in real time, but on my machine, set-argb-pixels takes 189.35±1.3 >> msec to run for a 500x500 image, which means I'm limited to displaying >> frames at ~5fps. >> >> Here's a toy benchmark to test set-argb-pixels: >> https://gist.github.com/4a5661dfad984cfdab19 >> >> There are some very simple bottlenecks that I've started to address: >> >> 1. It turns out that the references to b&w? and alpha-channel-local? for >> each pixel are slow slow slow. Making them local variables drops the >> time down to 124.8±1.0msec. This three-line change gives a speedup >> factor of about ~1.5 >> >> 2. Using unsafe functions everywhere (unsafe-bytes-ref and friends, >> unsafe-fx+ and friends) drops it further to 67.05±0.6msec, which is a >> speedup factor of ~2.82 over the original on my machine >> >> A pull request for the above is at >> https://github.com/plt/racket/pull/199 >> >> Now, if we can assume that the input bytes already contain pre-clipped, >> premultiplied data, we don't really have to loop through each pixel. If >> we copy each row using copy-bytes!, that drops the function to 9.55±6.1 >> msec (!) which is a speedup factor of ~20x over the original. >> >> The problem with that is on my little-endian machine, Cairo expects the >> input data in BGRA format, not RGBA, so the colors look wrong. Alas, >> this is why Racket's doing all the byte swizzling manually. >> >> Is there a fast native way of switching the endianness of a byte vector >> assumed to contain 32-bit ints? Or some way to do what we want? >> >> If there's a way to do this, this could make playing simple >> low-resolution videos from Racket pretty feasible. >> >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
