Re: Are these all really true ?

>===== ACADEMIC ASSUMPTIONS - ARE THESE ALL REALLY TRUE ? ======


>* Memory is free, speed is what is worth optimizing.


Most contemporary operating systems use some variation on paged
virtual memory. This means space and time interact. As the amount
of memory you use _ very actively_ increases, there comes a point
where page thrashing occurs and the speed of the memory (and the
whole computation) drops by orders of magnitude.


So algorithms that seem to gain speed at the cost of space can
suddenly turn out horribly slow. I see this a lot in compilerish
programs which have huge masses of linked data structure. Working on
making the data structure smaller can result in a significant speed
increase. Not that it's necessarily at all easy to do.


Now the same phenomenon is also occurring in the on-chip cache,
off-chip cache, physical ram memory hierarchy, as well as the ram to
paged backing store hierarchy.


So, if you want to optimize speed, act like memory, beyond a point,
is expensive.


Rodney Bates
bates@salsv3.boeing.com
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