
Ask HN: What are your thoughts on the Mill CPU? - deepnotderp
Do you think that it will be a technical and&#x2F;or commercial success?<p>Personally I think that they dance around the two issues that have historically plagued VLIWs, static branch prediction&#x2F;accurate trace scheduling and stochastic memory latency.<p>What&#x27;re your thoughts?
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j-pb
I like the Ideas.

I believe that it can exist in a world dominated by entirely different
architectures when I see it build into something.

Maybe with the decline of ever smaller processes and less new fabs being build
the current ones become cheap enough to make wildly alternative chip designs
viable. We'll see.

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mhkool
The Mill CPU is very innovative and I think that it will have the "Mach 3" as
promised in one of their videos: 3 times higher IPC. It has also a lot of
features that will make it more efficient. OS calls, function calls, function
returns, loops are all faster. And the cache is faster since there is no
address translation.

So I think the Mill CPU has a high potential. But progress is slow and as far
as I understood there is no big investor (yet). For good competition and
better consumer proces I would like to see that a non-US investor helps Mill
Computing Inc.

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deepnotderp
Where does the cache have no address translation? If anything I would expect
that a killer for them would be that their caches are more expensive with
9-bit bytes.

And the "Mach 3" strikes me as nothing but a skewed pipeline (albeit maybe
with some minor modifications).

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mhkool
The cache works with virtual addresses but the memory with real addresse, so
when there is a cache miss and memory is accessed, the translation is done.
Extra bits per cache byte is nothing new: CPUs already have ECC bits.
Explaining Mach 3 is not easy in a few sentences since it is based on a few
new concepts so I recommend to see the video to learn about it.

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deepnotderp
I've watched the video several times and conferred with a few people familiar
with Denver. It's quite similar to a skewed pipe, why do you say it's anything
different?

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mhkool
I am not familiar with skewed pipes, but nonetheless the properties of the
belt and the special Mill instructions like _retire_ gives the opportunity for
a new class of pipelining not seen before and with a 3 times higher IPC.

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deepnotderp
The belt is indeed interesting, but note that it precludes OoO should they
choose to add it at a later time. I'm not entirely optimistic about the belt
because basically the same thing exists on the Mali gpu (albeit not publicly
documented to the best of my knowledge) and it suffers from saturation issues.

The issue with ilp extraction for VLIW compilers has historically been the
difficulty of drawing accurate traces through branches to get sufficient ilp.
Nothing the mill has shown so far addresses this.

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mhkool
time will tell :-)

