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by ltbarcly3 13 days ago
Why can't anyone build a performant RISC-V cpu?

The SpacemiT K3 seems to be the fastest available right now, and it's basically a joke. https://www.phoronix.com/review/spacemit-k3-pico-itx/3

I'm starting to get the feeling that there is something fundamentally broken in the RISC-V specification that fundamentally limits performance.
4 comments
dismalaf 13 days ago
https://tenstorrent.com/newsroom/tenstorrent-sets-new-perfor...

It's already happening.

If you mean for consumer hardware though, it'll probably be a few more years. Even ARM hasn't really taken off for Windows. Hard to overcome developer inertia for consumer devices, whereas for datacenters it's easier as all the apps are custom anyway.

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brucehoult 13 days ago
> Why can't anyone build a performant RISC-V cpu?

"They" can, and are. Many "they"s.

> I'm starting to get the feeling that there is something fundamentally broken in the RISC-V specification that fundamentally limits performance.

Then you are at loggerheads with many legendary ISA and chip designers.

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tliltocatl 13 days ago
> I'm starting to get the feeling that there is something fundamentally broken in the RISC-V specification that fundamentally limits performance.

RISC-V is an objectively bad ISA design (a resell of MIPS dropping some of the most ominous features, then trying fix the code density issue with billions of extensions), but x86 is way worse and it didn't prevent Intel from making performant implementations. And RISC-V is certainly not bad in the way that would limit performance (well, maybe code density, but it's not the major issue).

The reality is, ISA matters very little for CPU performance. What really matters a lot is the memory subsystem and interconnect. I. e. good DRAM controllers IP are pricey - way more pricey than ARM cores AFAIK. Not a problem unique to RISC-V - i. e. Altera memory controllers used to be shit as well, not sure if Intel changed anything.

And another issue - there doesn't seem to be all that much money in CPUs any more. Look at ARM's history with high-performance sector.

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snvzz 13 days ago
>trying fix the code density issue with billions of extensions

Not sure what you're on about.

By the time the spec was first ratified (2019), RV64GC was already the densest 64bit ISA, and it's not even close.

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tliltocatl 12 days ago
> the densest 64bit ISA

Because all other 64-bits were designed with no regard to density too. On 32-bit (and thats where density matters) it's sill worse then Thumb even with compression extension.

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snvzz 12 days ago
>On 32-bit (and thats where density matters) it's sill worse then Thumb even with compression extension.

Thumb is really good, but this is no longer true as of late 2021, when bitmanip and the code reduction extensions landed.

RISC-V is currently the most dense 64bit ISA, as well as the most dense 32bit ISA.

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snvzz 13 days ago
>and it's basically a joke.

Look up the fab process node and die area of that chip, and think again.

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ltbarcly3 13 days ago
I get that they can make super tiny cheap processors, but why isn't there one faster than a 4 year old raspberry pi?
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