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.