ljh2

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ML is now automating parts of chip R&D. How big a deal is this?
Answer by ljh2Jun 11, 2021121

Just made this account to answer this. Source: I've worked in physical design/VLSI and CPU verification, and pretty regularly deal with RTL.

TL;DR - You're right-- it's not a big deal, but it simultaneously means more and less than you think.

The Problem

Jump to "What It Means" if you already understand the problem.

First, let me talk about about the purpose of floorplanning. The author's mention it a little bit, but it's worth repeating.

Placement optimizations of this form appear in a wide range of science and engineering applications, including hardware design, city planning, vaccine testing and distribution, and cerebral cortex layout.

Much like a city, an SoC (system-on-chip) has lots of agents that transfer data to each other. If a mayor has to get to city hall, the library, the post office, the locksmith, the school, the burger joint, etc., how do you best place the buildings to get the shortest path to each of them? Suppose suddenly the librarian wants to first go to school, then the post office, and also a burger because they have 20% off. How do you position that requirement along with the mayor's requirement? Do you prioritize the mayor? What if he wants a burger too? What if it's not guaranteed the number of paths the mayor will take before returning to city hall Etc. etc.

As you probably know, placement in general is an NP-complete problem. Tools for this exist, and/or you can do it manually, but much like city planning, it gets very complicated very fast. These tools (if you wanna sound cool, call them PnR tools (place-and-route)) take foooreeever to run (it's quite common to let a tool run for a week) and are critical in the holistic design lifecycle-- more on that later.

Enter this paper. Honestly, they don't do any revolutionary stuff-- CNNs, ReLu, weight adjustment-- or rather, it's revolutionary because it's applied to PnR for the first time that I've seen at least (which, in hindsight, is pretty obvious. Pulling up the GUI for the tool, it's literally just a grid, exactly like a city, with its own centers and everything. Still cool nevertheless). 

Let's talk about results!

I don't know how to do tables in comments, so bear with the formatting-- here are the results for one test they did:

Note: I left out "Congestion" and  "wire length" because those are metrics that tbh don't really matter 

MethodTimingTotal area (µm 2 )Total power (W)
(wns)(tns)
RePlAce374233.71,693,1393.70
Manual13647.61,680,7903.74
Our method8423.31,681,7673.59

Don't worry what wns and tns exactly mean (here are a few resources). Just know that they are essentially a measure of how short a "path" is between "buildings". The smaller it is, the better, because it means our mayor can travel less distance to get his burger.

Area and power are relatively explanatory-- essentially, how big is your city + all the roads you've built, and how much energy does it take to run it all. Again, the smaller the better.

What It Means

These are good results! We've just built roads that are twice as short vs. our manual methods! (23.3 vs. 47.6). But, I want to provide my opinion for why it's even worse than you think (i.e., I don't even think it would provide a 1% increase in perf, much in the same way that increasing CPU GHz doesn't do that much-- it's inherently limited), but also much better

For why it's worse-- consider again city planning. Suppose we take this to the extreme and the burger joint, library, post office, etc are all literally inside the same building as City Hall (i.e. no roads exist). First, his arteries will certainly get clogged passing by a McDonalds, but ultimately-- How much performance/time saved does the mayor really save?

I would argue that, while it depends on how convoluted the city was initially, there's a limit to how much you can shrink the roads and place the buildings. While these planning efforts are very much important to strive for, it's not the real bottleneck.

Furthermore, what if this travel time was time simultaneously being well-spent already? For instance-- perhaps he checked his emails walking to the post office. Maybe he called his mother. Maybe he brought his meeting notes to practice a speech. The point is-- this travel time is not really saved: just reallocated.

Note: CPUs do this a lot, e.g. while a memory request is occurring, they just switch to do some other tasks. This is also (to vastly oversimplify) essentially why frequency scaling no longer had immense payoffs as it did 30 years ago.

Now that I've killed your enthusiasm, let me tell you why it's also better than you think with this quote.

We show that our method can generate chip floorplans that are comparable or superior to human experts in under six hours, whereas humans take months to produce acceptable floorplans for modern accelerators

I mentioned earlier that designers heavily rely on PnR tools not only prior to tapeout, but as tools to iterate over (e.g. can I mux this more efficiently? Do I really need this logic in the critical path? Can this "building" be shifted over? etc.) As these tools take longer as our designs become more complex, it ultimately results in a longer feedback loop-- again, a week sometimes-- and personally, I really like instant gratification, so it's definitely a bit annoying.

And this is why it's potentially better-- it's indicative of a step towards freeing up resources of what I feel is a massive cost to many semiconductor companies. Not just for better and tighter feedback loops, but because these PnR/physical design/EDA tool teams are massive. Like, hundreds of people sometimes. And these people ultimately have the final signoff for lots of tapeouts, and determine timelines for hardware companies.

Go 5 years in the future, and give them a tool that improves engineer productivity 100x? Honestly, that'd be insane. For me personally, but also for my colleagues. (Honestly, not sure what I'd do with that extra time. I currently just cook stuff while I'm blocked- :) )

So, that's why I think it's both better and worse than you think.