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Date: 9 Aug 93 04:47:43 GMT
From: slinky.cs.nyu.edu!slinky.cs.nyu.edu!nobody@nyu.edu  (Robert Dewar)
Subject: Re: Query about monitor (passive) task optimization
Message-ID: <244ktf$61@schonberg.cs.nyu.edu>
List-Id: <comp.lang.ada>

Jonothan Shilling asks why people do not want automatic task optimization.
After all, he asks, it only makes programs run faster.

Well the point is that in realtime programs, running faster is not typically
a requirement. The requirement is that all tasks complete their assigned
tasks on schedule. General optimizations that may or may not speed up some
tasks, depending on what the compiler can figure out, are not much help to
a programmer who must demonstrate that the program he writes will meet its
deadlines.

Imagine a worst case. Suppose a compiler used some probabilistic algorithm
to estimate how it should optimize tasks. This is not unreasonable. The
compiler has incomplete information about the program being compiled, so
game theory would suggest that an optimizal strategy would be a mixed
stratecy involving probabilistic choices. Well such a compiler would be
unusable in real-time contexts -- every time you compiled, you might find
that different tasks were optimized and the program would change its
execution characteristics and perhaps correctness.
\
On the other hand, this compiler would, on average, execute Ada programs
faster than a compiler which did not do such probabistic optimization.

The question is: are there applications which use tasking, and are basically
interested in overall speed throughput, rather than simply that tasks
meet their deadlines. Well certainly one can construct examples, but I 
suspect that in real life such examples are few and far between.

Anyone care to report experience with applications of this type (i.e. ones
where the issue is not meeting deadlines, but just generally fast execution.
Remember they must also be applications where tasking performance is
in fact important to overall performance).

		Robert