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Linus Torvalds on AVX512

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Re: Linus Torvalds on AVX512

#91

Post by sgillen »

azalemeth wrote: post #52
topspin wrote: post #49

Earlier quoted context omitted.

He has the history correct. Most of the CPUs that x86 beat in the market had superior FP performance; SPARCs, Alphas, PA-RISC, Itanium, etc. > When's the last time he actually did anything with a computer? According to Linus he completes about 30 pull requests a day. Some multiple of that in kernel builds. His $1900 32 core Threadripper speeds that process a great deal and FP contributes little to nothing. Today peop…

Speaking as someone who does scientific computing all day long, in part with FEM simulations, even for me AVX512 isn't usually worth it in terms of wall-clock time.

Speaking as someone else who does scientific computing all day, taking away vectorized operations would kill my performance completely.

Re: Linus Torvalds on AVX512

#92

Post by gridlockd »

AnthonyMouse wrote: post #83
gridlockd wrote: post #81

Earlier quoted context omitted.

Bulldozer significantly outperformed Sandy Bridge on the workloads which it was designed to be good at, which is multi-threaded integer workloads, like compiling the Linux kernel. https://www.phoronix.com/scan.php?page=article&item=amd_fx81... https://www.phoronix.com/scan.php?page=article&item=amd_fx83... If Linus' attitude of "I'd rather have more cores" and "FP doesn't really matter" were representative of market…

Are we looking at the same benchmarks? In the first they're comparing an 8-core Bulldozer to Sandy Bridge with 4 cores and no hyperthreading and it's basically even, sometimes it wins by a small margin on the threaded ones. In the second the 3770K has 4 cores with hyperthreading and that makes it look even worse. If they were actually getting twice the integer performance per module as Intel was getting per core then…

You have to look for them, but there are benchmarks where AMD outperforms significantly. I cant find the Linux compilation benchmark now, but the difference was not small.

The Bulldozer really did have a big advantage in integer throughput per dollar, but that does not translate to a 2x speedup in pretty much any benchmark. FP throughput on the other hand shows up a lot.

Re: Linus Torvalds on AVX512

#93

Post by ethelward »

kmbriedis wrote: post #68
TinkersW wrote: post #66

Earlier quoted context omitted.

None of those are both portable and widely available on end user machines, which is needed for games CUDA seems nice, but being Nvidia only makes it a total dead end.

I believe the ML community will strongly disagree. CUDA is everything

Because the academic ML community does not care about shipping product to end users not equipped in nVidia.

Re: Linus Torvalds on AVX512

#94

Post by fancyfredbot »

AVX512 is both integer and floating point, not just FP, so this rant about FP comes across as ill informed.

Despite that I'd agree most people probably see no benefit from these units today. But that could change. For workloads with parallelism, wide SIMD is very efficient - more so than multiple threads anyway. The only way to get people to write vector code is to have vector processing available. Once it's ubiquitously available people might code for it and the benefits may become more apparent.

Re: Linus Torvalds on AVX512

#95

Post by bsder »

abainbridge wrote: post #12

What are the forces in chip design that are at play here? Over the last 10-15 years, fabs have continued to fit more and more logic gates per unit area, but haven't reduced the power consumption per gate as much. As a result, if you fill your modern chip with compute gates, you cannot use them all at once because the chip will melt. Or at least you can't have them all running at max clock rates. One solution is to in…

> What are the forces in chip design that are at play here?

The "weak form" of Moore's Law--"Performance doubles every 12-18 months"--is dead and buried.

The "strong form" of Moore's Law is still active--"Transistor cost halves every 12-18 months".

This means that you can't make the primary paths any faster. So, all you can do is add functionality and pray that someone magically can make that functionality relevant to the primary use cases.

Re: Linus Torvalds on AVX512

#96

Post by acqq »

em500 wrote: post #72
michaelt wrote: post #43

Earlier quoted context omitted.

> Over the last 10-15 years, fabs have continued to fit more and more logic gates per unit area, but haven't reduced the power consumption per gate as much. Kids these days get 8 cores for a 100W TDP. When I was a boy, 100W got you a single core. And you didn't get dynamic frequency scaling, so it'd be putting out that heat all the time . (We also had to walk to school barefoot in the snow, uphill both ways)

You must be young. Home PC CPUs from my youth drew only single digit watts. They didn't require any fan until the Pentium.

Indeed:

386, introduced 1985:

http://www.cpu-world.com/CPUs/80386/Intel-A80386-16.html

Typical/Maximum power dissipation: 1.85 Watt / 2.3 Watt

And even no Pentium III 1999-2003 needed more than around 30 W:

https://en.wikipedia.org/wiki/List_of_Intel_Pentium_III_micr...

Re: Linus Torvalds on AVX512

#97

Post by magicalhippo »

CamperBob2 wrote: post #86
ksec wrote: post #78

Earlier quoted context omitted.

What he said and I quote; >AVX2 is much more than enough.

Funny, that's not the same as the quote I responded to. Something about nobody caring about improving FP performance outside of benchmarks. Which of us is quoting Linus accurately? It can't be both of us, unless Linus is slipping into his dotage. Also: "640K is enough for anyone." "Who needs MMX? Just give me more superscalar execution units." "Who needs 3DNow? Nobody uses floats. Integer SIMD is fine." "Who needs SS…

I think you read it wrong, slightly helped by either Linus misspeaking or Phoronix misquoting him. The "matter" in "and it matter not one iota" clearly should be past tense.

As I see it, what he's saying is that back in the day, the majority of those buying CPUs did not care about FP code. And he thinks that today the same is true of AVX-512, the majority of those that buy CPUs don't care about AVX-512.

Re: Linus Torvalds on AVX512

#98

Post by tails4e »

amelius wrote: post #56
tails4e wrote: post #29

Earlier quoted context omitted.

Jim Keller had an interesting talk recently [1] about ways of doing parallel processing to better us the billions of transistors we have - assuming the task is parallelizable. There's the scalar core (i.e the basic CPU) which is easy to program realtively. Then a scalar core with vector instructions - difficult to program efficiently. Then there are arrays of scalar cores, i.e. GPUs, so relatively easy to program aga…

How does this solve the power problem that GP is talking about?

The power problem is solved by having cores more suited to a task. A CPU is completely general, but power inefficient. Dedicated HW is as efficient as it gets, but in the extreme is not flexible and only does one task well. With loads of extra silicon available, we can now use that for more specific engines/accelerators and of course not all of these would be active at once. So in a way the scaling / density does allow us to get more efficiency in some cases. The trick is finding the balance for a given process node.

Re: Linus Torvalds on AVX512

#99

Post by slavik81 »

TinkersW wrote: post #66
pjmlp wrote: post #60

Earlier quoted context omitted.

MSL, CUDA and SYSCL are C++ with extra topping. Also one of the reasons CUDA won developer love is that it fully embraced polyglot programming on the GPU.

None of those are both portable and widely available on end user machines, which is needed for games CUDA seems nice, but being Nvidia only makes it a total dead end.

Disclaimer: I work on AMD ROCm, but my opinions are my own.

There's also HIP[1], which can be used as a thin wrapper around CUDA, or with the ROCm backend on AMD platforms. It doesn't yet match CUDA in either breadth of features or maturity, but it's getting closer every day.

[1]: https://github.com/ROCm-Developer-Tools/HIP

Re: Linus Torvalds on AVX512

#100

Post by pjmlp »

ethelward wrote: post #93
kmbriedis wrote: post #68

Earlier quoted context omitted.

I believe the ML community will strongly disagree. CUDA is everything

Because the academic ML community does not care about shipping product to end users not equipped in nVidia.

Except SYSCL also works on AMD and Intel, and also has a CUDA backend, but apparently you missed that part.

In what concerns commercial uses of CUDA, Hollywood doesn't seem to have any problem with it, nor the car manufacturers with Jetson.

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