A problem is slowing down the CPU frequency significantly when AVX-512 is involved, e.g. https://en.wikichip.org/wiki/intel/xeon_gold/6262v this, which usually cancels out the benefit in the Real World (tm).
Removing characters from strings faster with AVX-512
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Re: Removing characters from strings faster with AVX-512
#12Earlier quoted context omitted.
My point is that vector instructions are fundamentally necessary and thus "what does it signal" evaluates to "nothing surprising". Sure, REP STOSB/MOVSB make for a very compact memset/memcpy, but their performance varies depending on CPU feature flags, so you're going to want multiple codepaths anyway. And vector instructions are vastly more flexible than just those two. Also, I have not met developers who expect AVX…
> vector instructions are fundamentally necessary For which percentage of users? > AMD is actually adding AVX-512 Which is irrelevant to in-market support for that instruction set.
> For which percentage of users?
Anyone using software that benefits from vector instructions. That includes a variety of compression, search, and image processing algorithms. Your JPEG decompression library might be using SSE2 or Neon. All high-end processors have included some form of vector instruction for like 20+ years now. Even the processor in my old eBook reader has the ARM Neon instructions.
Re: Removing characters from strings faster with AVX-512
#13Earlier quoted context omitted.
Why is a large speedup from vectors surprising? Considering that the energy required for scheduling/dispatching an instruction on OoO cores dwarfs that of the actual operation (add/mul etc), amortizing over multiple elements (=SIMD) is an obvious win.
Where do I say that the speedup is surprising? My question is whether Intel investing in AVX-512 is wise, given that: -Most existing code is not aware of AVX anyway; -Developers are especially wary of AVX-512, since they expect it to be discontinued soon. Consequently, wouldn't Intel be better off by using the silicon dedicated to AVX-512 to speed up instruction patterns that are actually used?
AVX-512 is not going to be discontinued. Intel's reticence/struggling with having it on desktop is irritating but it's here to stay on servers for a long time.
Writing code for a specific SIMD instruction set is non-trivial, but most code will get some benefit by being compiled for the right ISA. You don't get the really fancy instructions because the pattern matching in the compiler isn't very intelligent but quite a lot of stuff is going to benefit by magic.
Even without cutting people without some AVX off, you can have a fast/slow path fairly easily.
Re: Removing characters from strings faster with AVX-512
#14A problem is slowing down the CPU frequency significantly when AVX-512 is involved, e.g. https://en.wikichip.org/wiki/intel/xeon_gold/6262v this, which usually cancels out the benefit in the Real World (tm).
Intel has been trying to reduce the penalty for AVX-512, and barring that, advertise that there is no penalty. Most things on Ice Lake run fine with 256 bit vectors, but Skylake and earlier really needed 128 bit or narrower if you weren't doing serious vector math.
Forget about 512 bit vectors or FMAs.
Re: Removing characters from strings faster with AVX-512
#15A problem is slowing down the CPU frequency significantly when AVX-512 is involved, e.g. https://en.wikichip.org/wiki/intel/xeon_gold/6262v this, which usually cancels out the benefit in the Real World (tm).
I think this is less of a problem on newer CPUs: https://travisdowns.github.io/blog/2020/08/19/icl-avx512-fre...
Re: Removing characters from strings faster with AVX-512
#16A problem is slowing down the CPU frequency significantly when AVX-512 is involved, e.g. https://en.wikichip.org/wiki/intel/xeon_gold/6262v this, which usually cancels out the benefit in the Real World (tm).
Unless someone has data for the latest Intel chips (i.e. sapphire rapids) showing the opposite I'm inclined to think this is a meme from 2016/7 that needs to go the way of the dodo.
Re: Removing characters from strings faster with AVX-512
#17Cool performance enhancement, with an accompanying implementation in a real-world library ( https://github.com/lemire/despacer ). Still, what does it signal that vector extensions are required to get better string performance on x86? Wouldn't it be better if Intel invested their AVX transistor budget into simply making existing REPB prefixes a lot faster?
AVX-512 is an elegant, powerful, flexible set of masked vector instructions that is useful for many purposes. For example, low-cost neural net inference ( https://NN-512.com ). To suggest that Intel and AMD should instead make "existing REPB prefixes a lot faster" is missing the big picture. The masked compression instructions (one of which is used in Lemire's article) are endlessly useful, not just for stripping spa…
Many people seem to think AVX-512 is just wider AVX, which is a shame.
NN-512 is cool. I think the Go code is pretty ugly but I like the concept of the compiler a lot.
Re: Removing characters from strings faster with AVX-512
#18I prefer AMDs approach that allows them to put more cores on the die instead of supporting a rarely used instruction set.
Re: Removing characters from strings faster with AVX-512
#19A problem is slowing down the CPU frequency significantly when AVX-512 is involved, e.g. https://en.wikichip.org/wiki/intel/xeon_gold/6262v this, which usually cancels out the benefit in the Real World (tm).
I think this is less of a problem on newer CPUs: https://travisdowns.github.io/blog/2020/08/19/icl-avx512-fre...
Those are client CPUs, which have very different behavior around power management than server parts. However, AVX downclocking has mostly gone away with ice lake and hopefully sapphire rapids does away with it permanently (except on 512 bit vectors).
Re: Removing characters from strings faster with AVX-512
#20Please correct me if I'm wrong, but wouldn't we normally scale these things instead on a GPU?