/usr/man2/cat.3/OPENSSL_instrument_bus.3.Z

NAME

       OPENSSL_instrument_bus, OPENSSL_instrument_bus2 - instrument references
       to memory bus

SYNOPSIS

        #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

DESCRIPTION

       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 addi-
       tion probes are added to vector elements in atomic or interlocked man-
       ner, which should contribute additional noise on multi-processor sys-
       tems. 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 peri-
       ods 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

       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.

1.0.2t                            2019-09-10         OPENSSL_instrument_bus(3)
See also OPENSSL_instrument_bus2(3)