Re: [RFT v2 1/3] random: add rte_rand_float()

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 25 May 2022 14:45:37 +0000
Mattias Rönnblom <mattias.ronnblom@ericsson.com> wrote:

> On 2022-05-25 00:18, Stephen Hemminger wrote:
> > The PIE code and other applications can benefit from having a
> > fast way to get a random floating point value. This new function
> > is equivalent to erand48_r in the standard library.
> >   
> 
> Seems like a good addition to the API.
> 
> > Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> > ---
> >   app/test/test_rand_perf.c              |  7 +++++++
> >   doc/guides/rel_notes/release_22_07.rst |  5 +++++
> >   lib/eal/common/rte_random.c            | 21 +++++++++++++++++++++
> >   lib/eal/include/rte_random.h           | 17 +++++++++++++++++
> >   lib/eal/version.map                    |  1 +
> >   5 files changed, 51 insertions(+)
> > 
> > diff --git a/app/test/test_rand_perf.c b/app/test/test_rand_perf.c
> > index fe797ebfa1ca..f3602da5b2d4 100644
> > --- a/app/test/test_rand_perf.c
> > +++ b/app/test/test_rand_perf.c
> > @@ -20,6 +20,7 @@ static volatile uint64_t vsum;
> >   
> >   enum rand_type {
> >   	rand_type_64,
> > +	rand_type_float,
> >   	rand_type_bounded_best_case,
> >   	rand_type_bounded_worst_case
> >   };
> > @@ -30,6 +31,8 @@ rand_type_desc(enum rand_type rand_type)
> >   	switch (rand_type) {
> >   	case rand_type_64:
> >   		return "Full 64-bit [rte_rand()]";
> > +	case rand_type_float:
> > +		return "Floating point [rte_rand_float()]";
> >   	case rand_type_bounded_best_case:
> >   		return "Bounded average best-case [rte_rand_max()]";
> >   	case rand_type_bounded_worst_case:
> > @@ -55,6 +58,9 @@ test_rand_perf_type(enum rand_type rand_type)
> >   		case rand_type_64:
> >   			sum += rte_rand();
> >   			break;
> > +		case rand_type_float:
> > +			sum += rte_rand_float() * UINT64_MAX;
> > +			break;
> >   		case rand_type_bounded_best_case:
> >   			sum += rte_rand_max(BEST_CASE_BOUND);
> >   			break;
> > @@ -83,6 +89,7 @@ test_rand_perf(void)
> >   	printf("Pseudo-random number generation latencies:\n");
> >   
> >   	test_rand_perf_type(rand_type_64);
> > +	test_rand_perf_type(rand_type_float);
> >   	test_rand_perf_type(rand_type_bounded_best_case);
> >   	test_rand_perf_type(rand_type_bounded_worst_case);
> >   
> > diff --git a/doc/guides/rel_notes/release_22_07.rst b/doc/guides/rel_notes/release_22_07.rst
> > index e49cacecefd4..128d4fca85b3 100644
> > --- a/doc/guides/rel_notes/release_22_07.rst
> > +++ b/doc/guides/rel_notes/release_22_07.rst
> > @@ -104,6 +104,11 @@ New Features
> >     * ``RTE_EVENT_QUEUE_ATTR_WEIGHT``
> >     * ``RTE_EVENT_QUEUE_ATTR_AFFINITY``
> >   
> > +* ** Added function get random floating point number.**
> > +
> > +  Added the function ``rte_rand_float()`` to provide a pseudo-random
> > +  floating point number.
> > +
> >   
> >   Removed Items
> >   -------------
> > diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
> > index 4535cc980cec..024c3c41dc16 100644
> > --- a/lib/eal/common/rte_random.c
> > +++ b/lib/eal/common/rte_random.c
> > @@ -6,6 +6,7 @@
> >   #include <x86intrin.h>
> >   #endif
> >   #include <unistd.h>
> > +#include <ieee754.h>
> >     
> 
> Is this API available in FreeBSD? In Windows?
> 
> >   #include <rte_branch_prediction.h>
> >   #include <rte_cycles.h>
> > @@ -173,6 +174,26 @@ rte_rand_max(uint64_t upper_bound)
> >   	return res;
> >   }
> >   
> > +double
> > +rte_rand_float(void)
> > +{
> > +	struct rte_rand_state *state = __rte_rand_get_state();
> > +	union ieee754_double u = {
> > +		.ieee = {
> > +			.negative = 0,
> > +			.exponent = IEEE754_DOUBLE_BIAS,
> > +		},
> > +	};
> > +	uint64_t val;
> > +
> > +	/* Take 64 bit random value and put it into the mantissa */
> > +	val = __rte_rand_lfsr258(state);
> > +	u.ieee.mantissa0 = val >> 32;	/* only 20 bits used */
> > +	u.ieee.mantissa1 = (uint32_t)val;  
> 
> Why do you have a cast here, and not for mantissa0? Both will incur a 
> 64->32 conversion, assuming unsigned int is 32-bit (which it is on all 
> platforms DPDK supports?).

Yes, cast there is unnecessary. A little concerned that some compiler
might complain about loss of precision.

> 
> The naive implementation (i.e., (double)rte_rand() / (double)UINT64_MAX) 
> would cost a floating point multiplication. That's why you access the 
> underlying IEEE754 bits directly. Correct?

Yes, avoiding floating point math, and directly setting bits keeps full precision.