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SubjectRe: Linux 2.2.18 release notes
> > ... added basic support for the Pentium IV.
> How is the Pentium IV more advanced than the Pentium III, other than
> speed? Why would LInux care about a 1500 MHz clock or 400 MHz bus
> speed? Just treat the PIV as a faster PIII.

It all sounds so simple, right? Several small things to worry about:

- CPU identification and categorization. The P4 reports itself as CPU
family 15 rather than family 6 like PPro, PII and PIII. Thus, Linux
code that tests for certain features by saying if(cpufamily==6)
didn't notice that the P4 would work. Also there was at least one
format string like sprintf(buf, "i%d86", cpufamily) which is supposed
to print "i686" but on the P4 would print "i1586"....

- metrics -- L1 cacheline size is the important one: you align array
elements to this size when you want a per-cpu array, so that multiple
CPUs do not share a cacheline for accessing their "own" structure.
Proper alignment avoids "cacheline ping-pong", as it's called,
whenever two CPUs need to access "their" element of the same array at
the same time.

- as to the MHz -- there was a wraparound bug if your CPU is faster
than 2 GHz (highest signed 32-bit int), which isn't a problem today
but will be tomorrow.

- Tigran's microcode driver -- some small changes were made so that it
could be used for P4's.

- maybe they'll need to patch lm_sensors to accommodate the increased
temperature range since the P4 runs so hot. (: (:

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