Lots of concurrency
Andrew C. Greenberg
werdna at mucow.com
Thu Jan 1 08:09:24 UTC 1970
I fear this debate seems to be taking an odd twist into irrelevancy.
While it may be true (indeed unsurprising) that human cognition can be
modelled most appropriately using concurrency; indeed of a promiscuous
nature, I do not see how this indicates or counterindicates that a
concurrent model is the best model for human cognition to specify the
execution of system.
However self-aware we may be, such awareness of the means how we think
does not make us think best using that means. As others have noted,
concurrency is hard. it is hard, not only because languages do not
facilitate concurrent programming (though many do), but because
concurrency (or its abstraction, indeterminacy) is hard. It is trivial
to write a plausible concurrent program that is broken, and very, very
hard to find and diagnose the bases and source of a bug.
This is not to say that promiscuous determinacy doesn't also add
complexity -- it most certainly does. An example as trivial as the swap
operation of two values x and y, which is invariably modelled using a
t := x; x := y; y := t
introduces complexity unrelated to the problem. Indeed, Edsger Dijkstra
emphasized the problem of overdetermining code, favoring therefor the
language construct of the concurrent assignment, so swap can be
x,y := y,x
leaving the more detailed sequencing to the "system" to sort out. The
problem here is that such nondetermincy is well-constrained and easily
implemented in worst case. The sequencing and protection of shared
resources in concurrent programs is far more subtle and problematic.
My point is that while it is apparent that overdetermining code
sequentially often introduces levels of detail that precludes more
elegant expression of correct code, and sometimes distracts from best
ways to articulate the code, so, too, does expression of concurrency
introduce subtle unstated bugs and requires stating degrees of
sequencing, which are often harder to express than merely specifying the
sequencing in all its gory detail.
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