Friday, September 11, 2009

Of Muffins and Megahertz

Some readers have indicated, offline, that they liked the car salesman dialog about multicore systems that appeared in What Multicore Really Means. So I thought it might be interesting to relate the actual incident that prompted the miles- to muffins-per-hour performance switch I used.

If you haven't read that post, be warned that what follows will be more meaningful if you do.

It was inspired by a presentation to upper-level management of a 6-month-plus study of what was going on in the silicon concerning clock rate, and what if anything could be done about it. This occurred several years ago. I was involved, but not charged with providing any of the key slides. Well, OK, not one of my slides ended up being used.

It of course began with the usual "here's the team, here's how hard we worked" introduction.

First content chart: a cloud of data points collected from all over the industry that showed performance – specifically SPECINT, an integer benchmark – keeling over. It showed a big, obvious switch from the usual rise in performance, with a rough curve fit, breaking to a much lower predicted performance increase from now on. Pretty impressive. The obvious conclusion: Something major has happened. Things are different. There is big trouble.

Now, there's a rule for executive presentations: Never show a problem without proposing a solution. (Kind of like never letting a crisis go to waste.) So,

Second chart: a very similar-looking cloud of data points, sailing on at the usual growth rate for many years to come – labeled as multiprocessor (MP) results, what the industry would do in response. Yay, no problem! It's all right! We just keep on going! MP is the future! Lots of the rest of the pitch was about various forms of MP, from normal to bizarre.

Small print on second chart: It graphed SPECRATE. Not SPECINT.

SPECINT is a single-processor measure of performance. SPECRATE is, basically, how many completely separate SPECINTs you can do at once. Like, say, instead of the response time of PowerPoint, you get the incredibly useful measure of how many different PowerPoint slides you can modify at the same time. Or you change from miles per hour to muffins per hour.

Nothing on any slide or in any verbal statements referred to the difference. The chart makers - mostly high-level silicon technology experts - knew the difference, at least in theory. At least some of them did. I know others definitely did not in any meaningful sense.

At any event, throughout the entire rest of the presentation they displayed no inclination to inform anybody what it really meant. They didn't even distinguish the good result: typical server tasks can in general make really good use of parallelism. (See IT Departments Should NOT Fear Multicore.)

I was aghast. I couldn't believe that would be presented, like that, no matter what political positioning was going on. But I "knew better" than to say anything. Those charts were a result not just of data mining the industry for performance data but of data mining the company politically to get something that would reflect best on everybody involved. Speak up, and you get told that you don't know the big picture.

My opinion about feeding nonsense to anybody should be obvious from this blog. I don't think I'm totally blind in the political spectrum, but hey, guys, come on. That's blatant.

One hopes that the people who were the target knew enough to know the difference. I suspect that the whole point of the exercise, from their point of view, was just to really, firmly, nail down the point that the first chart – SPECINT keeling over – was a physical fact, and not just one of the regularly-scheduled pitches from the silicon folks for more development funds because they were in trouble. The target audience probably stopped paying attention after that first slide.

I don't mean to imply above that the gents who are responsible for the physical silicon don't regularly didn't have real problems; they do. But this situation was a problem of a whole different dimension.

It still is.

1 comment:

kme said...

This reminds me a _lot_ of this commentary from back in 1996, on bandwidth versus latency:

Particularly this part:

On the other hand, If you had to transport 1500 people and you only had one aeroplane to do it, the 747 could do it in three trips where the 737 would take ten, so you might say the Boeing 747 can transport large numbers of people three times faster than a Boeing 737, but you would never say that a Boeing 747 is three times faster than a Boeing 737.

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