Add a function rte_rand_max() which generates an 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.ronnb...@ericsson.com>
---
lib/librte_eal/common/include/rte_random.h | 16 ++++++++++++++++
lib/librte_eal/common/rte_random.c | 20 ++++++++++++++++++++
lib/librte_eal/rte_eal_version.map | 1 +
3 files changed, 37 insertions(+)
diff --git a/lib/librte_eal/common/include/rte_random.h
b/lib/librte_eal/common/include/rte_random.h
index 66dfe8ae7..053912168 100644
--- a/lib/librte_eal/common/include/rte_random.h
+++ b/lib/librte_eal/common/include/rte_random.h
@@ -47,6 +47,22 @@ rte_srand(uint64_t seedval);
uint64_t
rte_rand(void);
+/**
+ * Generates a 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 288e7b8c5..bf9240470 100644
--- a/lib/librte_eal/common/rte_random.c
+++ b/lib/librte_eal/common/rte_random.c
@@ -131,6 +131,26 @@ rte_rand(void)
return __rte_rand_lfsr258(state);
}
+uint64_t __rte_experimental
+rte_rand_max(uint64_t upper_bound)
+{
+ uint8_t zeros;
+ uint64_t mask = ~((uint64_t)0);
+ 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 0d60668fa..8f5b8c6a6 100644
--- a/lib/librte_eal/rte_eal_version.map
+++ b/lib/librte_eal/rte_eal_version.map
@@ -367,6 +367,7 @@ EXPERIMENTAL {
rte_mp_sendmsg;
rte_option_register;
rte_rand;
+ rte_rand_max;
rte_realloc_socket;
rte_service_lcore_attr_get;
rte_service_lcore_attr_reset_all;
--
2.17.1
