The road to Zettalinux [LWN.net]

The road to Zettalinux

Posted Sep 16, 2022 14:13 UTC (Fri) by ianmcc (subscriber, #88379)
In reply to: The road to Zettalinux by mathstuf
Parent article: The road to Zettalinux

Right, so what is wanted is a decorated or tagged pointer, and hardware support for that requires that the addressable space is smaller than the pointer size. Don't pitch it as needing more than 64 bit address sizes, pitch it as needing hardware support for decorated pointers.

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The road to Zettalinux

Posted Sep 17, 2022 21:41 UTC (Sat) by willy (subscriber, #9762) [Link] (2 responses)

The presentation calls for 128-bit registers in CPUs with fairly limited ALU support (addition, subtraction, shifts). That's what we need to avoid LFS2 and HIGHMEM

The road to Zettalinux

Posted Sep 19, 2022 9:35 UTC (Mon) by arnd (subscriber, #8866) [Link] (1 responses)

I think there is a conflict between the ideas of making 'long' 128-bit wide, using the same C calling conventions as 128-bit user space, and not requiring 128-bit multiplication and division to be fast.

We'll probably keep having this discussion for a while before even the basic type model is decided, including input from more than just kernel developers as well as actual benchmarking of user space software. My guess is that we'll end up with an LLP128 model (128 bit long long, 64 bit long) in both user space and kernel space, as is proposed in the draft riscv128 specs, which in turn means more invasive kernel changes but keeps the types compatible and does not require expensive 128-bit ALUs.

The road to Zettalinux

Posted Sep 19, 2022 16:49 UTC (Mon) by willy (subscriber, #9762) [Link]

I do think we'll end up with different conventions in userspace and kernel space. That's OK, as long as we have an API that can describe what's going on.

You make a good point that 128b x 128b is going to have to work. However, there are algorithms for this kind of multiplication & division that allow for early termination if the top bits are zero.