[erlang-questions] erlang-questions Digest, Vol 17, Issue 41

John Haugeland <>
Thu Oct 9 23:53:31 CEST 2008


>
> For example, if each of your player processes has a handle to each other
>>>
>> player process, spin up a process for a given match instead, and give the
>> player processes a hook to that and nothing else.
>>
>
> How does this work? Can you elaborate a bit?


Every mailing list needs ascii art!

If it used to be

  A---B
 /|\ /|\
C-+-*-+-D   (plus connections I can't draw
 \|/ \|/     from A-D, B-C, D-E, C-F)
  E---F

Then change it to

     Z
    /|\
   // \\
  / | | \
 /\ | | /\
A B C D E F

Instead of (6*5)=30 pids, you're now tracking (6*1)+6=12 pids.  The savings
goes up the larger the games get, because in the old fully connected network
you need N*(N-1) connections per game, whereas in the new network you need
2N connections per game, and exponential vs linear growth, and rada rada
rada.





>
>  Bang: 7/8 or so savings
>> on PID space, and probably a significant speed increase despite the extra
>> messaging, since you're going to get so many more correct branch
>> predictions
>> and cache wins.
>>
>
> And how do you measure cache wins and branch predictions with Erlang?


Ancient wisdom.  When you've been programming 20+ years, you can smell
what's going to work and what isn't.  And, I know, for someone who says
"intuition is worthless, profile it" as much as I do, that's kind of
hypocritical in the way that the ocean is kind of wet, but I haven't really
taken the time to learn Erlang's profiling situation yet, so it's the best
I've got.

But, I mean, when you drop the size of a block of data by 80-85%, you're
just going to keep a lot more of it in cache, and when the data has stronger
locality, it's more likely to get a good branch prediction.  That's just the
nature of modern branch prediction algorithms.  There's a fascinating (if
somewhat poorly written) discussion of these issues in the original Judy
Tree papers by that guy from Hewlett Packard.

  - John
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