The implementation of rte_rand() returns only 62 bits of
pseudo-randomness, because the underlying calls to lrand48()
"return non-negative long integers uniformly distributed
between 0 and 2^31."
We have written a potential fix, but before we spend more
time testing and refining it, I wanted to check with you
guys.
We switched to using the reentrant versions of [ls]rand48,
and maintain per-lcore state. We need ~2.06 calls to
lrand48_r(), per call to rte_rand().
Do you agree with the approach we've taken in this patch?
Thanks,
Robert
---
lib/librte_eal/common/include/rte_random.h | 33 +++++++++++++++++++++++++--
lib/librte_eal/linuxapp/eal/eal_thread.c | 7 ++++++
2 files changed, 37 insertions(+), 3 deletions(-)
diff --git a/lib/librte_eal/common/include/rte_random.h
b/lib/librte_eal/common/include/rte_random.h
index 24ae836..b9248cd 100644
--- a/lib/librte_eal/common/include/rte_random.h
+++ b/lib/librte_eal/common/include/rte_random.h
@@ -46,6 +46,17 @@ extern "C" {
#include <stdint.h>
#include <stdlib.h>
+#include <rte_per_lcore.h>
+#include <rte_branch_prediction.h>
+
+struct rte_rand_data {
+ struct drand48_data _dr48;
+ uint32_t _hi_bits;
+ uint8_t _bits_left;
+};
+
+RTE_DECLARE_PER_LCORE(struct rte_rand_data, _rand_data);
+
/**
* Seed the pseudo-random generator.
@@ -60,7 +71,7 @@ extern "C" {
static inline void
rte_srand(uint64_t seedval)
{
- srand48((long unsigned int)seedval);
+ srand48_r((long unsigned int)seedval, &RTE_PER_LCORE(_rand_data)._dr48);
}
/**
@@ -76,10 +87,26 @@ rte_srand(uint64_t seedval)
static inline uint64_t
rte_rand(void)
{
+ struct rte_rand_data *rd = &RTE_PER_LCORE(_rand_data);
uint64_t val;
- val = lrand48();
+ uint32_t hi_bits;
+ long int result;
+
+ if (unlikely(rd->_bits_left < 2)) {
+ lrand48_r(&rd->_dr48, &result);
+ rd->_hi_bits |= (uint32_t)result << (1 - rd->_bits_left);
+ rd->_bits_left += 31;
+ }
+
+ hi_bits = rd->_hi_bits;
+ lrand48_r(&rd->_dr48, &result);
+ val = (uint32_t)result | (hi_bits & 0x8000000);
val <<= 32;
- val += lrand48();
+ hi_bits <<= 1;
+ lrand48_r(&rd->_dr48, &result);
+ val |= (uint32_t)result | (hi_bits & 0x8000000);
+ rd->_hi_bits = hi_bits << 1;
+ rd->_bits_left -= 2;
return val;
}
diff --git a/lib/librte_eal/linuxapp/eal/eal_thread.c
b/lib/librte_eal/linuxapp/eal/eal_thread.c
index 5635c7d..08e7f72 100644
--- a/lib/librte_eal/linuxapp/eal/eal_thread.c
+++ b/lib/librte_eal/linuxapp/eal/eal_thread.c
@@ -52,6 +52,8 @@
#include <rte_eal.h>
#include <rte_per_lcore.h>
#include <rte_lcore.h>
+#include <rte_cycles.h>
+#include <rte_random.h>
#include "eal_private.h"
#include "eal_thread.h"
@@ -59,6 +61,8 @@
RTE_DEFINE_PER_LCORE(unsigned, _lcore_id) = LCORE_ID_ANY;
RTE_DEFINE_PER_LCORE(unsigned, _socket_id) = (unsigned)SOCKET_ID_ANY;
RTE_DEFINE_PER_LCORE(rte_cpuset_t, _cpuset);
+RTE_DEFINE_PER_LCORE(struct rte_rand_data, _rand_data);
+
/*
* Send a message to a slave lcore identified by slave_id to call a
@@ -147,6 +151,9 @@ eal_thread_loop(__attribute__((unused)) void *arg)
/* set the lcore ID in per-lcore memory area */
RTE_PER_LCORE(_lcore_id) = lcore_id;
+ /* seed per-lcore PRNG */
+ rte_srand(rte_rdtsc());
+
/* set CPU affinity */
if (eal_thread_set_affinity() < 0)
rte_panic("cannot set affinity\n");
--
1.7.1
