REORG: tools: promote the debug PRNG to more general use as a statistical one
We frequently need to access a simple and fast PRNG for statistical
purposes. The debug_prng() function did exactly this using a xorshift
generator but its use was limited to debug only. Let's move this to
tools.h and tools.c to make it accessible everywhere. Since it needs to
be fast, its state is thread-local. An initialization function starts a
different initial value for each thread for better distribution.
diff --git a/src/debug.c b/src/debug.c
index 209199a..914c80d 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -42,16 +42,6 @@
volatile unsigned long threads_to_dump = 0;
unsigned int debug_commands_issued = 0;
-/* Xorshift RNGs from http://www.jstatsoft.org/v08/i14/paper */
-static THREAD_LOCAL unsigned int y = 2463534242U;
-static unsigned int debug_prng()
-{
- y ^= y << 13;
- y ^= y >> 17;
- y ^= y << 5;
- return y;
-}
-
/* dumps a backtrace of the current thread that is appended to buffer <buf>.
* Lines are prefixed with the string <prefix> which may be empty (used for
* indenting). It is recommended to use this at a function's tail so that
@@ -748,7 +738,7 @@
t->expire = tick_add(now_ms, inter);
/* half of the calls will wake up another entry */
- rnd = debug_prng();
+ rnd = statistical_prng();
if (rnd & 1) {
rnd >>= 1;
rnd %= tctx[0];
@@ -770,7 +760,7 @@
/* wake up two random entries */
for (i = 0; i < 2; i++) {
- rnd = debug_prng() % tctx[0];
+ rnd = statistical_prng() % tctx[0];
rnd = tctx[rnd + 2];
if (rnd & 1)
diff --git a/src/tools.c b/src/tools.c
index 1c43354..d3ee426 100644
--- a/src/tools.c
+++ b/src/tools.c
@@ -78,6 +78,12 @@
THREAD_LOCAL char quoted_str[NB_QSTR][QSTR_SIZE + 1];
THREAD_LOCAL int quoted_idx = 0;
+/* thread-local PRNG state. It's modified to start from a different sequence
+ * on all threads upon startup. It must not be used or anything beyond getting
+ * statistical values as it's 100% predictable.
+ */
+THREAD_LOCAL unsigned int statistical_prng_state = 2463534242U;
+
/*
* unsigned long long ASCII representation
*
@@ -5362,6 +5368,16 @@
*out++ = 0;
return pos - shift;
}
+
+static int init_tools_per_thread()
+{
+ /* Let's make each thread start from a different position */
+ statistical_prng_state += tid * MAX_THREADS;
+ if (!statistical_prng_state)
+ statistical_prng_state++;
+ return 1;
+}
+REGISTER_PER_THREAD_INIT(init_tools_per_thread);
/*
* Local variables: