Manual:OPENSSL instrument bus(3)
OPENSSL_instrument_bus, OPENSSL_instrument_bus2 - instrument references to memory bus
#ifdef OPENSSL_CPUID_OBJ size_t OPENSSL_instrument_bus (int *vector,size_t num); size_t OPENSSL_instrument_bus2(int *vector,size_t num,size_t max); #endif
It was empirically found that timings of references to primary memory are subject to irregular, apparently non-deterministic variations. The subroutines in question instrument these references for purposes of gathering entropy for random number generator. In order to make it bus-bound a 'flush cache line' instruction is used between probes. In addition probes are added to vector elements in atomic or interlocked manner, which should contribute additional noise on multi-processor systems. This also means that vector[num] should be zeroed upon invocation (if you want to retrieve actual probe values).
OPENSSL_instrument_bus performs num probes and records the number of oscillator cycles every probe took.
OPENSSL_instrument_bus2 on the other hand accumulates consecutive probes with the same value, i.e. in a way it records duration of periods when probe values appeared deterministic. The subroutine performs at most max probes in attempt to fill the vector[num], with max value of 0 meaning "as many as it takes."
Return value of 0 indicates that CPU is not capable of performing the benchmark, either because oscillator counter or 'flush cache line' is not available on current platform. For reference, on x86 'flush cache line' was introduced with the SSE2 extensions.
Otherwise number of recorded values is returned.