Some comments on the submission from 16 hours back: https://news.ycombinator.com/item?id=33916480
Can they get merged?
I've merged the threads into the current one. It's later, but it's still on the front page.
Knight’s Landing: Atom with AVX-512
21–30 of 37 posts
Re: Knight’s Landing: Atom with AVX-512
#22[deleted]
Re: Knight’s Landing: Atom with AVX-512
#23Great in-depth article. One especially interesting data point relevant to ongoing discussions about AVX-512 area cost: 40% of 2.93 mm2 per core is AVX-512, so 1.14 mm2. This is a large fraction, but as the article says the core is basically a minimum wrapper around the vector unit, with rather weak L1i/branch predictor/store buffers. Let's put that in the context of modern chips. 14nm density was 44.67 MTr/mm2 so tha…
> give us more cores instead
That should be a pretty obvious conclusion--higher-clocked cores is better than wider cores, and wider cores is better than more cores.
Communication cost between concurrency domains for superscalar/supervector: min Communication cost between concurrency domains for multiprocessors: min 10-100ns
Re: Knight’s Landing: Atom with AVX-512
#24I wonder how well this would run a raytracer, aren't those pretty branchy?
Re: Knight’s Landing: Atom with AVX-512
#25I wonder how well this would run a raytracer, aren't those pretty branchy?
Fairly well, Intel's Embree raytracing kernels have support for Xeon Phi, although going more than 8-wide with incoherent rays (i.e. pathtracing) starts to require more and more sorting to batch things up for decent utilisation...
Re: Knight’s Landing: Atom with AVX-512
#26I wonder how well this would run a raytracer, aren't those pretty branchy?
Fairly well, Intel's Embree raytracing kernels have support for Xeon Phi, although going more than 8-wide with incoherent rays (i.e. pathtracing) starts to require more and more sorting to batch things up for decent utilisation...
Would you mind giving an easy explanation of the difference between raytracing and pathtracing? I find it a bit confusing.
Re: Knight’s Landing: Atom with AVX-512
#27Great in-depth article. One especially interesting data point relevant to ongoing discussions about AVX-512 area cost: 40% of 2.93 mm2 per core is AVX-512, so 1.14 mm2. This is a large fraction, but as the article says the core is basically a minimum wrapper around the vector unit, with rather weak L1i/branch predictor/store buffers. Let's put that in the context of modern chips. 14nm density was 44.67 MTr/mm2 so tha…
Knight’s Landing AVX-512 units run at about 1.5 GHz at peak (even when only a few are in use.)
The ones in Sapphire Rapids run 2x+ faster. Likely there is more pipelining and many more transistors used to reach those clock speeds.
Re: Knight’s Landing: Atom with AVX-512
#28Earlier quoted context omitted.
Fairly well, Intel's Embree raytracing kernels have support for Xeon Phi, although going more than 8-wide with incoherent rays (i.e. pathtracing) starts to require more and more sorting to batch things up for decent utilisation...
Would you mind giving an easy explanation of the difference between raytracing and pathtracing? I find it a bit confusing.
Ray tracing is typically incapable of diffuse reflections. It goes from the camera and traverses geometry until hitting something, possibly experiencing a few mirror/shiny reflections.
Path tracing is usually bi-directional (from both light and camera) and uses something like Metropolis-Hastimgs to sample more efficiently than brute force. Our still requires many samples per pixel but the sampling nature means it can naturally support the random bouncing of diffuse reflections. Between bounces the is typically still ray tracing, though, unless it's some special volumetric material like Jade (the green jewel rock) or fog or (the contents of) a glass of milk.
Compare e.g. POV-Ray to LuxRenderer.
Re: Knight’s Landing: Atom with AVX-512
#29Great in-depth article. One especially interesting data point relevant to ongoing discussions about AVX-512 area cost: 40% of 2.93 mm2 per core is AVX-512, so 1.14 mm2. This is a large fraction, but as the article says the core is basically a minimum wrapper around the vector unit, with rather weak L1i/branch predictor/store buffers. Let's put that in the context of modern chips. 14nm density was 44.67 MTr/mm2 so tha…
Good analysis. It's also worth pointing out that this is for 2x 512 bit FMA, which is more than client Ice/Tiger/Rocket lake or Zen4 have. Personally, I bought HEDT (Skylake-X and Cascadelake) because I wanted 2x 512 bit AVX512. Glad it's cheap in terms of area, and I'm hoping we'll get more options with great vector performance in the future.
Good point about the second FMA. I'm not certain it's the best tradeoff, Genoa only has two half-width FMA.
I share your hope for more focus on vectors. It's also up to us software devs, CPUs will not invest as heavily if we don't use it.
Re: Knight’s Landing: Atom with AVX-512
#30Great in-depth article. One especially interesting data point relevant to ongoing discussions about AVX-512 area cost: 40% of 2.93 mm2 per core is AVX-512, so 1.14 mm2. This is a large fraction, but as the article says the core is basically a minimum wrapper around the vector unit, with rather weak L1i/branch predictor/store buffers. Let's put that in the context of modern chips. 14nm density was 44.67 MTr/mm2 so tha…
Knight’s Landing AVX-512 units run at about 1.5 GHz at peak (even when only a few are in use.) The ones in Sapphire Rapids run 2x+ faster. Likely there is more pipelining and many more transistors used to reach those clock speeds.
Physical design is outside my area of expertise, but I understand that a large part of the increase may be due to improved fab. 15-20% speed improvement per node would account for the majority of the clock speed gain. Also, with smaller feature sizes comes the ability to trade away some of the density for further increases in the frequency, without actually requiring more transistors.