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0 pointsimtringued2mo ago0 comments

This feels like a pointless form of pendantry.

Okay, so we went from linear address spaces to partioned/disaggregated linear address spaces. This is hardly the victory you claim it is, because page sizes are increasing and thus the minimum addressable block of memory keeps increasing. Within a page everything is linear as usual.

The reason why linear address spaces are everywhere has to do with the fact that they are extremely cost effective and fast to implement in hardware. You can do prefix matching to check if an address is pointing at a specific hardware device and you can use multiplexers to address memory. Addresses can easily be encoded inside a single std_ulogic_vector. It's also possible to implement a Network-on-Chip architecture for your on-chip interconnect. It also makes caching easier, since you can translate the address into a cache entry.

When you add a scan chain to your flip flops, you're implicitly ordering your flip flops and thereby building an implicit linear address space.

There is also the fact that databases with auto incrementing integers as their primary keys use a logical linear address space, so the most obvious way to obtain a non-linear address space would require you to use randomly generated IDs instead. It seems like a huge amount of effort would have to be spent to get away from the idea of linear address spaces.

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nine_k2mo ago

We have a linear address space where we can map physical RAM and memory-mapped devices dynamically. Every core, at any given time, may have its own view of it. The current approach uses pretty coarse granularity, separating execution at the process level. The separation could be more granular.

The problem is the granularity of trust within the system. Were the MMU much faster, and TLB much larger (say, 128MiB of dedicated SRAM), the granularity might be pretty high, giving each function's stack a separate address space insulated from the rest of RAM. This is possible even now, just would be impractically slow.

Any hierarchical (tree-based) addressing scheme is equivalent to a linear addressing scheme, pick any tree traversal algorithm. Any locally-hierarchical addressing scheme seemingly can be implemented with (short) offsets in a linear address space; this is how most jumps in x64 and aarch64 are encoded, for instance.

latchup2mo ago

It has very little to do with trust and a lot to do with the realities of hardware implementation. Every interconnect has a minimum linear transfer granularity that properly utilizes its hardware links, dictated primarily by its physical link width and minimum efficient burst length. The larger this minimum granularity, the faster and more efficient moving data becomes. However, below this granularity, bandwidth and energy efficiency crater. Hence, reducing access granularity below this limit has disastrous consequences.

In fact, virtual memory is already a limiting factor to increasing minimum transfer size, as pages must be an efficient unit of exchange. Traditional 4 KiB pages are already smaller than what would be a good minimum transfer size; this is exactly why hardware designers push for larger pages (with Apple silicon forgoing 4 KiB page support entirely).

I cannot help but feel that many of these discussions are led astray by the misconceptions of people with an insufficient understanding of modern computer architecture.

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