Algorithms

Algorithms

XFreq is split in 3 algorithms :

1- Nehalem architecture and superior, based on the fixed performances counters

• step a- Initialize counters, write the MSR registers:
• IA32_PERF_GLOBAL_CTRL (0x38f)
• IA32_FIXED_CTR_CTRL (0x38d)
• step b- Read the MSR registers:
• IA32_FIXED_CTR0 (0x309)
• IA32_FIXED_CTR1(0x30a)
• IA32_FIXED_CTR2 (0x30b)
• IA32_TIME_STAMP_COUNTER (0x10)
• MSR_CORE_C3_RESIDENCY (0x3fc)
• MSR_CORE_C6_RESIDENCY(0x3fd)
• step c- Compute and display
• ratios: * Relative frequency ratio * IPS * IPC * CPI
• c-states: * Turbo [UCC] & [URC] * C0 * C1 * C3 * C6
• step d- Loop to step b

2- Core 2 algorithm, similar to the previous one, except that there is none MSR_CORE_C3_RESIDENCY and MSR_CORE_C6_RESIDENCY.

• step a- Initialize counters
• step b- Read the IA32_FIXED_CTR MSRs
• step c- only C0 and C1 states are taken into account.
• step d- Loop to step b

3- A fallback algorithm for Genuine architectures:

• step a- Read the MSR IA32_APERF (0xe8) , IA32_MPERF (0xe7) and IA32_TIME_STAMP_COUNTER (0x10)
• step b- Computes, displays C0 and C1 states only from values read in step a
• step c- Loop to step a

When the server starts and the processor signature is detected from CPUID, one of the 3 algorithms is selected then launched. If you noticed wrong counters values, you can select another algorithm from the Architectures table.