Wednesday, November 07

Geek

Readers Ask Questions

Mauser asks, regarding the article on The Superpermutation of Haruhi Suzumiya:
Why would you need to watch all the episodes in every possible order?
Well, true, perhaps you wouldn't.  We've seen Endless Eight.

But let's try another scenario: You're Intel, and your brand new CPU turns out to crash apparently randomly.  You call in your entire test team and they can reproduce the crash, about once per chip per week on average.

And some bright spark figures out that it happens seventeen cycles after you do an integer add, then an integer divide, then issue an AVX256 MADD, then a relative conditional branch, because if you do that exact sequence a register file port gets left in a stuck state and when the branch prediction finally gets resolved, BANG.

Only...  What about other sequences?  Is this the only problem?  If you patch it with a microcode update and systems keep right on crashing, you're not going to set sales records this quarter.

You want to test all possible sequences of instructions, one by one, as quickly as possible.  This theorem lets you do it an order of magnitude faster than a naive approach, and provides rules for generating the optimal sequence of instructions.

Posted by: Pixy Misa at 08:31 PM | Comments (4) | Add Comment | Trackbacks (Suck)
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1 So in other words, somebody having fun while looking at something whimsical, inadvertently added to our reality (i.e. our understanding of mathematics, the 'language of reality').  And the people doing it didn't even notice.  That is one of the most Haruhi things I've heard of.

Posted by: StargazerA5 at Thursday, November 08 2018 01:01 AM (Q7Wqc)

2 tl;dr: Haruhi leaves register ports in a stuck state once every 93 billion episodes.  Or something like that.

Posted by: kurt duncan at Thursday, November 08 2018 02:30 AM (noh54)

3 I don't think you'd want to use superpermutations for your proposed use. You'd want to allow for the same instruction used more than once in a sequence--that would be a De Bruijn sequence instead.

Posted by: Kayle at Thursday, November 08 2018 02:53 AM (TtvMc)

4 That's true, it's more a sort of "how this might be of use" optimisation example than a "this is what Intel actually does".  In either case, the instruction set is too large for it to be practical and you'd have to engage in fairly drastic pruning to get anything done.

Posted by: Pixy Misa at Thursday, November 08 2018 10:53 AM (PiXy!)

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Apple pies are delicious. But never mind apple pies. What colour is a green orange?




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