2017-11-10 22:13 GMT+01:00 James Darnley <jdarn...@obe.tv>: > On 2017-11-10 14:32, James Darnley wrote: > > I mentioned previously that using ZMM registers will cause the CPU to > > reduce its frequency. > > > > Gramner said on IRC that a user should spend 20-30% of time in > > AVX-512/ZMM code for it to be a net gain in speed. > > From ffmpeg-devel IRC on 2017-10-26 > >> https://lists.ffmpeg.org/pipermail/ffmpeg-devel-irc/ > 2017-October/004622.html > >> [18:49:26 CEST] <Gramner> J_Darnley: be aware that using zmm registers > induces significant frequency drops which reduces performance of everything > else, so if you want to use 512-bit vectors you better go all in on it to > make up for it. you probably want to spend at least 20-30% of overall > runtime in avx-512 code > >> [18:50:00 CEST] <Gramner> the alternative is to stay in 256-bit mode > and just leverage new instructions and opmasks > > > > This means any cycles you might save by using longer registers, fewer > > instructions, better instructions, whatever, will be lost because the > > frequency drops meaning it takes longer to execute overall. > > Some details about this can be found in one of Intel's documents: IntelĀ® > 64 and IA-32 Architectures Optimization Reference Manual > Order Number: 248966-038 > October 2017 > > https://software.intel.com/sites/default/files/managed/ > 9e/bc/64-ia-32-architectures-optimization-manual.pdf > Specifically section 15.26 "SKYLAKE SERVER POWER MANAGEMENT" > > Earlier on the ffmpeg-devel IRC channel I posted a link to Cloudflare's > blog in which they discuss the effects of running just a few (my words) > AVX-512/ZMM instructions. > > https://blog.cloudflare.com/on-the-dangers-of-intels-frequency-scaling/ > > In the worst cases on some of the new processors the frequency drop can > be 1GHz. In Cloudflare's case just spending about 2.5% of time in a > cryptography function using AVX-512 was causing a 10% drop in their > overall performance (requests served per second). > > After seeing this and the discussion on IRC I won't commit any of the > function patches. The functions are not very impressive and are likely > to make everything else slower. > > The IRC log should appear at the link below. > > https://lists.ffmpeg.org/pipermail/ffmpeg-devel-irc/ > 2017-November/004651.html > > > Thanks for the details explanations.
Martin _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel