PAT isn't about performance per se.
Until now, the kernel uses MTRR and while that's a tad slow, it's done once during X startup,
and pretty much static afterwards... (and "tad slow" is not human-noticable at all, it's way
The real problem is that the number of MTRR entries is very limited (8 on Intel CPUs for
example); this is in fact so few that BIOSes increasingly will need/use all 8 just to map the
existing system, so there's no new ones available for X in such a case..... PAT is the only
option then to get write-combinable memory.
So *if* you hit the "8 limit", it's a big performance thing, but if you don't hit the limit...
you'll not notice the difference.