HN Reader

Something in the article that I had to look up that might bother others. He uses the term 'DCT' in this sentence, but it's never defined in the article. AFAIK it stands for 'DRAM Memory Controller', but that could be an LLM hallucination. Running a web search defines it as Discrete Cosine Transform. :P

> "AMD’s BIOS and Kernel Developer’s Guide (BKDG) indicates there’s a 4-bit read pointer for a sideband signal FIFO between the GMC and DCT, so the “Garlic” link may have a queue with up to 16 entries."

Should maybe swap DCT in for MCT (memory controller)?

It is probably good that Chips and Cheese stays technical and objective. But, this was from the pre-Zen bad days of AMD, right? I wonder if a “where’d it all go wrong” post would be more interesting. Or, maybe more optimistically, “how’d this set things up for Zen.”

It's wild how much extra work was done to avoid coherency, yet share memory.

Ok, there's the first part, the Garlic bus, which gives the GPU its own access to the DRAM request controller, instead of going through the CPU's memory controller.

Since the GPU is mostly going to miss, it's great that it's not wasting energy trying to go to the CPU's cache. But it means if you do want to share memory now you need a whole other access path for the GPU to read from the CPU memory, even though it's literally the same RAM (but maybe different cache). So, add a new Onion link, that lets the GPU go through the crossbar, and get handled by the memory controller. And this one is slower.

Infinity Fabric seems conceptually so much easier, to keep things in sync. But the work to snoop the bus, to maintain coherency: it has to be pretty massive effort.

It's so so different a thing, but I wonder how AMD deal with coherency (or not?) on the 6 Memory Control Die (MCD) in the 6800xt GPU. Having separate chips whose job is to be cache and dram controller, that must need at least some understanding of who has what memory, that has to be wild.

One other comment, on:

> modern games struggle or won’t launch at all on Trinity, so I’ve selected a few older workloads

I wonder how many more games would run under Linux? Theres an absurd amount of work still going into the radeonsi driver. The driver just switched to the newer ACO compiler pipeline by default, last December, for example. That said, Trinity is (2012) using a (2010) TeraScale3 (gfx4). This is old! But the improvements have been ongoing, in a way commercial systems would unlikely to ever be; there's so many wins over such a long time; not compatibility but getting multi threaded driver support (2017) also comes to mind as a big leap! https://www.phoronix.com/news/RadeonSI-ACO-Default-Pre-GFX10 https://www.phoronix.com/news/RadeonSI-G3D-Threads https://www.google.com/search?q=site%3Aphoronix.com+radeonsi

I wonder how granular the breakdown/fallback modes are for running ; I suspect if there's an unsupportable feature somewhere in the graphics pipeline the whole pipeline will usually need to fallback to CPU rendering, but perhaps perhaps perhaps there's some ability to fill in some GPU features via CPU while running most of the pipeline on CPU (and not having the latency destroy everything, perhaps using that Onion link/cacheable host memory)?