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Context Switching: Difference between revisions
brendan should read more closely
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Because the hardware mechanism saves almost all of the CPU state it can be slower than is necessary. For example, when the CPU loads new segment registers it does all of the access and permission checks that are involved. As most modern operating systems don't use segmentation, loading the segment registers during context switches may be not be required, so for performance reasons these operating systems tend not to use the hardware context switching mechanism. Due to it not being used as much CPU manufacturers don't optimize CPUs for this method anymore (AFAIK). In addition the new 64 bit CPU's do not support hardware context switches when in 64 bit/long mode.
However, there was an interesting post on OSNews by Aage in July 2004, quantifying the amount of unavoidable hardware overhead involved in a context switch. It appears that the hardware overhead in a context switch on a modern P4 processor dwarfs the overhead involved in saving/loading registers (995ns of HW overhead vs 67ns to save/load registers). From this, it would appear that any performance gains from switching to software task switching would be minimal, amounting to no more than a few percentage points
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