MINOR: debug: add a trivial PRNG for scheduler stress-tests

Commit a5a447984 ("MINOR: debug: add "debug dev sched" to stress the
scheduler.") doesn't scale with threads because ha_random64() takes care
of being totally thread-safe for use with UUIDs. We don't need this for
the stress-testing functions, let's just implement a xorshift PRNG
instead. On 8 threads the performance jumped from 230k ctx/s with 96%
spent in ha_random64() to 14M ctx/s.

diff --git a/src/debug.c b/src/debug.c
index aaba9ca786be35dda2cc6d43104fac25334a6145..f83a7c018c74bbd465c40562cdc2499fe186e705 100644 (file)
--- a/src/debug.c
+++ b/src/debug.c
@@ -42,6 +42,17 @@
 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 = 2463534242;
+static unsigned int debug_prng()
+{
+
+        y ^= y << 13;
+        y ^= y >> 17;
+        y ^= y << 5;
+        return y;
+}
+
 /* Dumps to the buffer some known information for the desired thread, and
  * optionally extra info for the current thread. The dump will be appended to
  * the buffer, so the caller is responsible for preliminary initializing it.
@@ -684,7 +695,7 @@ static struct task *debug_task_handler(struct task *t, void *ctx, unsigned short
        t->expire = tick_add(now_ms, inter);
 
        /* half of the calls will wake up another entry */
-       rnd = ha_random64();
+       rnd = debug_prng();
        if (rnd & 1) {
                rnd >>= 1;
                rnd %= tctx[0];
@@ -706,7 +717,7 @@ static struct task *debug_tasklet_handler(struct task *t, void *ctx, unsigned sh
 
        /* wake up two random entries */
        for (i = 0; i < 2; i++) {
-               rnd = ha_random64() % tctx[0];
+               rnd = debug_prng() % tctx[0];
                rnd = tctx[rnd + 2];
 
                if (rnd & 1)