Wednesday, 1 May 2013

Reading comprehension, hUMA, NUMA, HSA, FSA, WTF?

I really need to find something better to do in my spare time than read ars "tech" nica and the like, but whilst doing a pass over the confusing front-page I came across an article about AMD's hUMA press. At least the front page isn't as bad as anandtech - i''m not sure what 'pipeline stories' are supposed to be, and to be honest i'm not sure why I bother reading a site which is full of computer case and psu reviews (ffs) and otherwise rather personally biased coverage of pretty random topics.

Anyway back to the arsetechnica piece. Pretty lazy article all round but I guess it summarised some of the points.

The real laff is with the comments.

Quite a few people seem to be getting "hUMA" confused with "NUMA". Hint: The N is for "NOT". Detail: Non-Unified-Memory-Architecture is exactly the opposite to Unified-Memory-Architecture which is the UMA part of the hUMA acronym.

NUMA is a way to add a lot of memory to a system with a lot of processors and not be bottlenecked by concurrent access issues (this is very much a good thing, it scales very well). UMA just makes the memory fast enough that the concurrent access shouldn't matter and then puts everything on the same memory ... (but it can't scale as well).

The rest of the comments just show that nobody knows what the 'h' means either. Probably understandable, it's a bloody horrid acronym and the article goes no way to explaining what's going on beyond the one set of slides in that press pack - however the information is readily available on AMD's site.

i.e. the h is for HSA, ... which is the other side of the coin. Another mouth-full at Hetereogenous Systems Architecture (off the top of my head, could be off a bit - i'm not a journalist).

In a nut-shell, AMD and the other HSA co-conspirators are working on turning their custom processors, DSPs, FPGAs, and GPUs into first-class CPU-compatible co-processors. They will all need to share the same virtual (and protected) address space that the CPU does. They will need to support a coherent cache (at some level, L2 at least). Obviously (like duh) this will require operating system support although apart from the CPU I would suspect it can just be hidden in the driver. Personally I hope the coherency isn't too fine-grained otherwise it will be a bottleneck on it's own.

And the other big part (from the last information I read on it at least) is that HSA uses a common assembly language/binary format/bytecode which can be re-targetted to different platforms cheaply, at run-time. So if the hardware provides the resources required, it will just run from a single compile. Although I suspect for performance it will have to target 'classes' of hardware, since to get good GPU performance you really need to write things very differently. I presume this will be based capability based on things like LDS memory.

Obviously AMD have to do this so that developers are able to target legacy Intel/PC hardware for free as well since neither Intel nor Nvidia are part of HSA - nor are they likely to be if they have any choice in the matter since it's such a big benefit to AMD's technology.

I think the commenters are also missing the point on just how much GPUs and CPUs have already converged. CPUs keep getting a wider MMX, as well as 'hyper-threading' and so on. And GPUs now have scalar units running the show, pre-emptive threading (in addition to the super-hyper threading they already have) and other processor features. The new GPUs will be capable of directly executing other languages like Java or Python or whatever - how those would handle vectorisation is another issue.

Anyway ... man, I hope they can pull it off. Right now working with a GPU it's like trying to solve every transport problem with a frieght-train. Sure you can get a lot of work done but it's not the best suited tool to every transport job - sometimes you can just walk. Like everything in the peecee wintel world getting to this point has been the product of throwing enough hardware and power at a problem until the architectural inefficiencies are inconsequential. This isn't good system design unless you're trying to sell the big hardware parts that drive it (i.e. you're intel).

The technology is great. The challenges are great. The wintel inertia which must be overcome is great too. The challenge of making the hardware easy enough to programme that all developers can take advantage of it ... is nigh on insurmountable.

With lambda's and the parallel collections Java could be a perfect fit. Well that language will be. With the JVM being so friggan complex, hopefully the implementation wont be a decade getting there as it was with cpus.

No comments: