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

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

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?).

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?

> +
> +	return u.d - 1.0;
> +}
> +
>   static uint64_t
>   __rte_random_initial_seed(void)
>   {
> diff --git a/lib/eal/include/rte_random.h b/lib/eal/include/rte_random.h
> index 29f5f1325a30..553beb2d9c6f 100644
> --- a/lib/eal/include/rte_random.h
> +++ b/lib/eal/include/rte_random.h
> @@ -65,6 +65,23 @@ rte_rand(void);
>   uint64_t
>   rte_rand_max(uint64_t upper_bound);
>   
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice
> + *
> + * Generates a pseudo-random floating point number.
> + *
> + * This function returns a nonnegative double-precison floating random
> + * number uniformly distributed over the interval [0.0, 1.0).
> + *
> + * If called from lcore threads, this function is thread-safe.
> + *
> + * @return
> + *   A pseudo-random value between 0 and 1.0.
> + */
> +__rte_experimental
> +double rte_rand_float(void);
> +

Newline after "double" missing.

I would call it something else than "float", in particular since it 
doesn't return "float" but a "double" type floating point value.

rte_drand() maybe? Short, but might be confused with rte_rand(), given 
the visual similarity. I still I would still prefer that over 
rte_rand_double().

>   #ifdef __cplusplus
>   }
>   #endif
> diff --git a/lib/eal/version.map b/lib/eal/version.map
> index d49e30bd042f..861906af2999 100644
> --- a/lib/eal/version.map
> +++ b/lib/eal/version.map
> @@ -422,6 +422,7 @@ EXPERIMENTAL {
>   	rte_intr_type_set;
>   
>   	# added in 22.07
> +        rte_rand_float;
>   	rte_thread_get_affinity_by_id;
>   	rte_thread_self;
>   	rte_thread_set_affinity_by_id;