[data from a 2.0 GHz Athlon64 in 32-bit mode]

PERFCTR INIT: vendor 2, family 15, model 4, stepping 8, clock 2000097 kHz
PERFCTR INIT: NITER == 64
PERFCTR INIT: loop overhead is 244 cycles
PERFCTR INIT: rdtsc cost is 19.8 cycles (1513 total)
PERFCTR INIT: rdpmc cost is 20.6 cycles (1565 total)
PERFCTR INIT: rdmsr (counter) cost is 52.3 cycles (3597 total)
PERFCTR INIT: rdmsr (evntsel) cost is 57.6 cycles (3932 total)
PERFCTR INIT: wrmsr (counter) cost is 74.8 cycles (5035 total)
PERFCTR INIT: wrmsr (evntsel) cost is 331.7 cycles (21473 total)
PERFCTR INIT: read cr4 cost is 9.2 cycles (835 total)
PERFCTR INIT: write cr4 cost is 69.9 cycles (4723 total)
PERFCTR INIT: write LVTPC cost is 24.5 cycles (1816 total)
PERFCTR INIT: sync_core cost is 167.3 cycles (10955 total)
perfctr: driver 2.6.9, cpu type AMD K8C at 2000097 kHz

[data from a 2.0 GHz Athlon64 in 64-bit mode]

PERFCTR INIT: vendor 2, family 15, model 4, stepping 8, clock 2000119 kHz
PERFCTR INIT: NITER == 64
PERFCTR INIT: loop overhead is 244 cycles
PERFCTR INIT: rdtsc cost is 12.6 cycles (1052 total)
PERFCTR INIT: rdpmc cost is 18.2 cycles (1412 total)
PERFCTR INIT: rdmsr (counter) cost is 55.1 cycles (3776 total)
PERFCTR INIT: rdmsr (evntsel) cost is 57.2 cycles (3907 total)
PERFCTR INIT: wrmsr (counter) cost is 73.5 cycles (4952 total)
PERFCTR INIT: wrmsr (evntsel) cost is 332.3 cycles (21512 total)
PERFCTR INIT: read cr4 cost is 6.0 cycles (629 total)
PERFCTR INIT: write cr4 cost is 68.7 cycles (4642 total)
PERFCTR INIT: write LVTPC cost is 24.7 cycles (1831 total)
PERFCTR INIT: sync_core cost is 163.9 cycles (10739 total)
perfctr: driver 2.6.9, cpu type AMD K8C at 2000119 kHz