/usr/man/cat.3/OPENSSL_instrument_bus2.3

OPENSSL_instrument_bus(3)    OpenSSL    OPENSSL_instrument_bus(3)

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 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

     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               Last change: 2019-09-10                    1

See also OPENSSL_instrument_bus(3)