[dpdk-dev] [RFC 3/3] Introduce random generator functions with upper bound
Mattias Rönnblom
mattias.ronnblom at ericsson.com
Mon Apr 8 14:30:29 CEST 2019
Add two functions rte_rand32_max() and rte_rand_max(), which generate
uniformly distributed pseudo-random number less than a user-specified
upper bound.
The commonly used pattern rte_rand() % SOME_VALUE, in addition to
being slow, also creates biased results if SOME_VALUE is not a power
of 2. This bias is very small for small SOME_VALUE, but increases
linearly with larger SOME_VALUE.
Signed-off-by: Mattias Rönnblom <mattias.ronnblom at ericsson.com>
---
lib/librte_eal/common/include/rte_random.h | 32 +++++++++++++++++
lib/librte_eal/common/rte_random.c | 40 ++++++++++++++++++++++
lib/librte_eal/rte_eal_version.map | 2 ++
3 files changed, 74 insertions(+)
diff --git a/lib/librte_eal/common/include/rte_random.h b/lib/librte_eal/common/include/rte_random.h
index a96fc7f96..b560b0a0d 100644
--- a/lib/librte_eal/common/include/rte_random.h
+++ b/lib/librte_eal/common/include/rte_random.h
@@ -45,6 +45,22 @@ rte_srand(uint64_t seedval);
uint32_t
rte_rand32(void);
+/**
+ * Generates an 32-bit pseudo-random number less than upper_bound.
+ *
+ * This function returns an uniformly distributed (unbiased) random
+ * number lower than a user-specified maximum value.
+ *
+ * If called from lcore threads, this function is thread-safe.
+ *
+ * @param upper_bound
+ * The upper bound of the generated number.
+ * @return
+ * A pseudo-random value between 0 and (upper_bound-1).
+ */
+uint32_t
+rte_rand32_max(uint32_t upper_bound);
+
/**
* Generates a 64-bit pseudo-random number.
*
@@ -56,6 +72,22 @@ rte_rand32(void);
uint64_t
rte_rand(void);
+/**
+ * Generates an 64-bit pseudo-random number less than upper_bound.
+ *
+ * This function returns an uniformly distributed (unbiased) random
+ * number lower than a user-specified maximum value.
+ *
+ * If called from lcore threads, this function is thread-safe.
+ *
+ * @param upper_bound
+ * The upper bound of the generated number.
+ * @return
+ * A pseudo-random value between 0 and (upper_bound-1).
+ */
+uint64_t
+rte_rand_max(uint64_t upper_bound);
+
#ifdef __cplusplus
}
#endif
diff --git a/lib/librte_eal/common/rte_random.c b/lib/librte_eal/common/rte_random.c
index c6dac2fc7..67d07d3ff 100644
--- a/lib/librte_eal/common/rte_random.c
+++ b/lib/librte_eal/common/rte_random.c
@@ -93,6 +93,26 @@ rte_rand32(void)
return __rte_rand32(state);
}
+uint32_t __rte_experimental
+rte_rand32_max(uint32_t upper_bound)
+{
+ uint32_t mask = ~((uint32_t)0);
+ uint8_t zeros;
+ uint32_t res;
+
+ if (upper_bound < 2)
+ return 0;
+
+ zeros = __builtin_clz(upper_bound);
+ mask >>= zeros;
+
+ do {
+ res = rte_rand32() & mask;
+ } while (unlikely(res >= upper_bound));
+
+ return res;
+}
+
uint64_t __rte_experimental
rte_rand(void)
{
@@ -108,6 +128,26 @@ rte_rand(void)
return low | (high << 32);
}
+uint64_t __rte_experimental
+rte_rand_max(uint64_t upper_bound)
+{
+ uint64_t mask = ~((uint64_t)0);
+ uint8_t zeros;
+ uint64_t res;
+
+ if (upper_bound < 2)
+ return 0;
+
+ zeros = __builtin_clzll(upper_bound);
+ mask >>= zeros;
+
+ do {
+ res = rte_rand() & mask;
+ } while (unlikely(res >= upper_bound));
+
+ return res;
+}
+
RTE_INIT(rte_rand_init)
{
rte_srand(rte_get_timer_cycles());
diff --git a/lib/librte_eal/rte_eal_version.map b/lib/librte_eal/rte_eal_version.map
index 027a22fe5..abdacb217 100644
--- a/lib/librte_eal/rte_eal_version.map
+++ b/lib/librte_eal/rte_eal_version.map
@@ -367,7 +367,9 @@ EXPERIMENTAL {
rte_mp_sendmsg;
rte_option_register;
rte_rand32;
+ rte_rand32_max;
rte_rand;
+ rte_rand_max;
rte_realloc_socket;
rte_service_lcore_attr_get;
rte_service_lcore_attr_reset_all;
--
2.17.1
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