Comment by glial

Comment by glial 4 days ago

7 replies

Apologies if this is a dumb question, but aren't all computations inherently serial? In that a Turing machine performs operations serially?

joe_the_user 3 days ago

Aren't all computations inherently serial?

No. "inherently serial" refers to problems that are specified serially and can't be spend up by parallel processing. The sum of a set of N numbers is an example of a problem that is not inherently serial. You can use parallel reduction to perform the computation in O(log(N)) time on an idealized parallel computer but it takes O(N) time on an idealized serial computer.

And, it turns, exactly which problems are really are inherently serial is somewhat challenging problem.

  • visarga 3 days ago

    > The sum of a set of N numbers is an example of a problem that is not inherently serial.

    But addition with floats (not reals) is non associative.

    • immibis 3 days ago

      They didn't say floats, and the sum of a set of floats is not uniquely defined as a float for the rain you stated, at least not without specifying a rounding mode. Most people use "round to whatever my naïve code happens to do" which has many correct answers. To add up a set of floats with only the usual 0.5ULP imprecision, yes, isn't trivial.

tromp 3 days ago

Turing Machines are just one of many computational models. Others offer more parallelism. Two examples:

In lambda calculus, disjoint redexes can be reduced in parallel.

And in interaction nets, all active pairs can be reduced in parallel [1].

[]1 https://en.wikipedia.org/wiki/Interaction_nets

ants_everywhere 3 days ago

You can model parallel computation by an arbitrary finite product of Turing machines. And then, yes, you can simulate that product on a single Turing machine. I think that's the sort of thing you have in mind?

But I'm not aware of what "inherently serial" means. The right idea likely involves talking about complexity classes. E.g. how efficiently does a single Turing machine simulate a product of Turing machines? An inherently serial computation would then be something like a problem where the simulation is significantly slower than running the machines in parallel.

ninetyninenine 3 days ago

Yeah it's talking about a new feature for LLMs where the output of an LLM is fed back in as input and done again and again and again and this produces way more accurate output.