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0 pointstripletao2mo ago0 comments

> The core of this is the Arrhenius rate, (A × T^n × exp(-E_a/(R×T))), which involves an exponentiation, a division, a multiplication, and an exponential. On a GPU, that's multiple SFU calls chained with ALU ops. In an EML tree, the whole expression compiles to a single tree that flows through the pipeline in one pass.

I think you're missing the reason why the GPU kicks you out of the fast path when you need that special function. The special function evaluation is fundamentally more expensive in energy, whether that cost is paid in area or time. Evaluation of the special functions with throughput similar to the arithmetic throughput would require much more area for the special functions, which for most computation isn't a good tradeoff. That's why the GPU's designers chose to make your exp2 slow.

Replacing everything with dozens of cascaded special functions makes everything uniform, but it's uniformly much worse. I feel like you're assuming that by parallelizing your "EML tree" in dedicated hardware that problem goes away; but area isn't free in either dollars or power, so it doesn't.

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tgtweak2mo ago· 1 in thread

There is a huge market for "its faster" at the cost of efficiency, but I don't think your claim that an EML hardware block would be inherently less inefficient than the same workload running on a GPU. If you think it would be, back it up with some numbers.

A 10-stage EML pipeline would be about the size of an avx-512 instruction block on a modern CPU, in the realm of ~0.1mm2 on a 5nm process node (collectively including the FMA units behind it), at it's entirety about 1% of the CPU die. None of this suggests that even a ~500 wide 10-stage EML pipeline would be consuming anywhere near the power of a modern datacenter GPU (which wastes a lot of it's energy moving things from memory to ALU to shader core...).

Not sure if you're arguing from a hypothetical position or practical one but you seem to be narrowing your argument to "well for simple math it's less efficient" but that's not the argument being made at all.

tripletaoOP2mo ago

> you seem to be narrowing your argument to "well for simple math it's less efficient" but that's not the argument being made at all.

What? Unless the thing you want to compute happens to be exactly that eml() function (no multiplication, no addition, no subtraction unless it's an exponential minus a log, etc.) or almost so, it is unquestionably less efficient. If you believe otherwise, then please provide the eml() implementation of a practically useful function of your choice (e.g. that Arrhenius rate). Then we can count the superfluous transcendental function evaluations vs. a conventional implementation, and try to understand what benefit could outweigh them.

> A 10-stage EML pipeline would be about the size of an avx-512 instruction block on a modern CPU

Can you explain where you got that conclusion? And what do you think a "10-stage EML pipeline" would be useful for? Remember that the multiply embedded in your Arrhenius rate is already 8 layers and 12 operations.

Also, can you confirm whether you're working with an LLM here? You're making a lot of unsupported and oddly specific claims that don't make sense to me, and I'm trying to understand where they're coming from.

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