Context switching (was: delay until and GNAT)

["Nick Roberts" <nickroberts@callnetuk.com>]

I'd just to add a little note on context switching.

[:1:]  On some processors, a 'fast' context switch can be achieved in just a
few CPU clock cycles (by simply switching register banks), which, on a
modern 'RISC' processor, could equate to a mere few picoseconds.  In
practice, obviously, this is a facility likely to be used only in HRT
embedded systems, or for fundamental system software only.

[:2:]  Many processor architectures today provide built-in support for
(normal) context switching, so that the operating system will usually have
very little to do with the speed of these context switches.  Switches can
generally be achieved within a few dozen memory clock cycles (typically
out-of-cache), which will be, for most modern microcomputers, in the
ballpark of 1 microsecond (+/-1oom).

[:3:]  Some operating systems (naming no names ;-) are amazingly slow at
context switches, taking many thousands of instructions to achieve just one
switch.

If you can get access to a high-resolution system clock from within a
compiled program, you can easily test the performance of a particular
system.  Try running under different loading conditions: 1 task; 10 tasks;
100 tasks; 1000 tasks.  (You may find the 1000 trial goes kerplunk.)  Come
to think of it, I might write a little Ada program to do this and post it on
comp.lang.ada for fun!

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Nick Roberts
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