On Fri, Jun 25, 2021 at 1:24 PM Alan Kelly <alanke...@google.com> wrote:
> On Fri, Jun 25, 2021 at 10:40 AM Lynne <d...@lynne.ee> wrote: > >> Jun 25, 2021, 09:54 by alankelly-at-google....@ffmpeg.org: >> >> > Broadwell and later and Zen3 and later have fast gather instructions. >> > --- >> > Gather requires between 9 and 12 cycles on Haswell, 5 to 7 on >> Broadwell, >> > and 2 to 5 on Skylake and newer. It is also slow on AMD before Zen 3. >> > libavutil/cpu.h | 2 ++ >> > libavutil/x86/cpu.c | 18 ++++++++++++++++-- >> > libavutil/x86/cpu.h | 1 + >> > 3 files changed, 19 insertions(+), 2 deletions(-) >> > >> >> No, we really don't need more FAST/SLOW flags, especially for >> something like this which is just fixable by _not_using_vgather_. >> Take a look at libavutil/x86/tx_float.asm, we only use vgather >> if it's guaranteed to either be faster for what we're gathering or >> is just as fast "slow". If neither is true, we use manual lookups, >> which is actually advantageous since for AVX2 we can interleave >> the lookups that happen in each lane. >> >> Even if we disregard this, I've extensively benchmarked vgather >> on Zen 3, Zen 2, Cascade Lake and Skylake, and there's hardly >> a great vgather improvement to be found in Zen 3 to justify >> using a new CPU flag for this. >> _______________________________________________ >> ffmpeg-devel mailing list >> ffmpeg-devel@ffmpeg.org >> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel >> >> To unsubscribe, visit link above, or email >> ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe". >> > > Thanks for your response. I'm not against finding a cleaner way of > enabling/disabling the code which will be protected by this flag. However, > the manual lookups solution proposed will not work in this case, the avx2 > version of hscale will only be faster if fast gathers are available, > otherwise, the ssse3 version should be used. > > I haven't got access to a Zen3 so I can't comment on the performance. I > have tested on a Zen 2 and it is slow. On Broadwell hscale avx2 is about > 10% faster than the ssse3 version and on Skylake about 40% faster, Haswell > has similar performance to Zen2. > > Is there a proxy which could be used for detecting Broadwell or Skylake > and later? AVX512 seems too strict as there are Skylake chips without > AVX512. Thanks > Hi, I will paste the performance figures from the thread for the other part of this patch here so that the justification for this flag is clearer: Skylake Haswell hscale_8_to_15_width4_ssse3 761.2 760 hscale_8_to_15_width4_avx2 468.7 957 hscale_8_to_15_width8_ssse3 1170.7 1032 hscale_8_to_15_width8_avx2 865.7 1979 hscale_8_to_15_width12_ssse3 2172.2 2472 hscale_8_to_15_width12_avx2 1245.7 2901 hscale_8_to_15_width16_ssse3 2244.2 2400 hscale_8_to_15_width16_avx2 1647.2 3681 As you can see, it is catastrophic on Haswell and older chips but the gains on Skylake are impressive. As I don't have performance figures for Zen 3, I can disable this feature on all cpus apart from Broadwell and later as you say that there is no worthwhile improvement on Zen3. Is this OK with you? Thanks _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
