> Another datapoint: I get 2.1 us on a Pentium/MMX 200 (SDRAM) with no
> extra processes. I get 8.1 us (best case) with 12 extra processes. So
> on this system we're down to 0.5 us per process.
> On a PPro 180 (EDO) I go from 4.5 us to 9.8 us, a cost of 0.44 us per
> process.
>
> Now, I don't know for sure what is the cause of the variability, but I
> suspect cache problems, and given that, the run queue costs don't seem
> unreasonable.
Aren't most of the variables referenced at the same addess modulo 4k since
they are in the task_state+stack structure ? In this case you repeatedly
access the same cache set, meaning that your code is a cache thrashing
benchmark every time you scan a queue with more elements than the L1 cache
associativity. (And maybe some variables are modified, causing copybacks
of dirty cache lines, I can't tell, not having access to the source
right now.)
Regards,
Gabriel.
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