Messages in this thread | | | Subject | Re: Scheduler still seems awful with x264, worse with patches | From | Mike Galbraith <> | Date | Thu, 10 May 2012 12:29:45 +0200 |
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On Wed, 2012-05-09 at 09:00 -0700, Jason Garrett-Glaser wrote: > Many months ago, the topic of CFS's inefficiencies with x264 came up > and some improvements were made, but BFS and Windows still stayed a > little bit in the lead. This seemed to be because of a mix of two > issues. Firstly, a combination of relatively short-lived jobs (x264 > uses a thread pool, so the actual threads are long-lived). Secondly, > in frame threads, heavy dependencies between threads, benefiting > greatly from a dumb scheduler. Thirdly, in sliced threads -- the > focus of this post -- the best scheduling approach is to simply spread > them throughout the cores and do nothing, so again, a dumb scheduler > will do the right thing. > > Recently I tried multithreading x264's lookahead for a customer. The > lookahead previously wasn't threaded, causing bottlenecks with many > cores and threads. I do my development mainly on Windows, and the > patch looked to be quite a success, with nice performance boosts for > many target use-cases. > > And then I ran it on Linux and it choked horribly. > > The patch is here: > https://github.com/DarkShikari/x264-devel/commit/99e830f1581eac3cf30f07b1d6c6c32bae1725c8 > . To replicate the test, simply test that version against the > previous version. My guess is the reason it chokes is that it > involves spawning even *shorter*-lived jobs than x264 typically does, > something that CFS seems to simply collapse on. > > Here's some stats from a recent kernel: > > SD encoding (before -> after patch): > CFS: 325.49 +/- 1.22 fps -> 251.68 +/- 2.32 fps > BFS: 334.94 +/- 0.59 fps -> 344.47 +/- 0.68 fps > > HD encoding (before -> after patch): > CFS: 39.05 +/- 0.22 fps -> 40.56 +/- 0.23 fps > BFS: 40.15 +/- 0.05 fps -> 44.89 +/- 0.05 fps > > As can be seen, the longer the threads live (the lower the fps), the > less horrific the penalty is. Furthermore, though I don't have > numbers, using schedtool -R -p 1 does basically as well as BFS in > eliminating the problem. Naturally, this is not really a solution as > it requires root. > > To replicate this test, a commandline like this should work on any > cached raw input file (a collection of free raw videos can be found > here if you don't like making your own: > http://media.xiph.org/video/derf/ ): > > ./x264 --preset superfast --tune zerolatency --threads X input -o /dev/null
On my Q6600 box, neither scheduler (identical configs) seems to like --tune zerolatency much.
# ultrafast x264 --quiet --no-progress --preset ultrafast --no-scenecut --sync-lookahead 0 --qp 20 -o /dev/null --threads $1 ./soccer_4cif.y4m x264 --quiet --no-progress --preset ultrafast --tune zerolatency --no-scenecut --sync-lookahead 0 --qp 20 -o /dev/null --threads $1 ./soccer_4cif.y4m x264 --quiet --no-progress --preset ultrafast --tune zerolatency -o /dev/null --threads $1 ./soccer_4cif.y4m
3.3.0-bfs 3.3.0-cfs marge:~/tmp # ./x264.sh 8
encoded 600 frames, 449.63 fps 400.20 fps encoded 600 frames, 355.00 fps 304.12 fps encoded 600 frames, 305.65 fps 267.25 fps
marge:~/tmp # schedctl -I ./x264.sh 8
encoded 600 frames, 475.00 fps 483.19 fps encoded 600 frames, 364.72 fps 278.37 fps encoded 600 frames, 311.69 fps 256.25 fps
marge:~/tmp # schedctl -R ./x264.sh 8
encoded 600 frames, 454.70 fps 489.00 fps encoded 600 frames, 358.83 fps 365.61 fps encoded 600 frames, 308.81 fps 310.46 fps
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