[Computer-go] pachi2

[Olivier Teytaud]

Hi and thanks for posting this interesting summary.
The surprising thing is that 128 machines are much worse than 64;
is this due to communication cost ? I would expect a stagnation more than
a decrease of performance, but maybe the master/slave system does not have
the logarithmic cost as a function of the number of nodes (whereas tree-like
communications
by averaging have a logarithmic number of communications per computaton
node) and therefore you have a significant communication overhead with big
number of computation nodes ?

For the bias point of view (your second paragraph below) I think we all have
the same scalability limits, unfortunately... we've tried many things for
including life&death analysis
without any success.

Best regards,
Olivier

In distributed mode doubling the number of machines initially gains
> approximately 50 elo (half a stone) up to 8 machines.  Above this we
> quickly hit a scalability limit and the best result so far is with 64
> machines; this is the configuration used for the KGS tournament (starting
> at round 4) and on KGS right now. 128 machines are currently much worse
> than 64.
>
> Preliminary analysis of the lost games shows that the current code
> has inherent scalability limits because the playouts are biased.
> When the playouts incorrectly judge the life status of a group,
> the results will be bad no matter how many cores and machines
> work on it. We are of course working on this to eliminate these
> scalability limits.
>
>
>
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