Add ticker_geom_t.

This lets a single ticker object drive events across a large number of different
tick streams while sharing state.

src/ticker.c

@@ -0,0 +1,32 @@
+#include "jemalloc/internal/jemalloc_preamble.h"
+#include "jemalloc/internal/jemalloc_internal_includes.h"
+
+/*
+ * To avoid using floating point math down core paths (still necessary because
+ * versions of the glibc dynamic loader that did not preserve xmm registers are
+ * still somewhat common, requiring us to be compilable with -mno-sse), and also
+ * to avoid generally expensive library calls, we use a precomputed table of
+ * values.  We want to sample U uniformly on [0, 1], and then compute
+ * ceil(log(u)/log(1-1/nticks)).  We're mostly interested in the case where
+ * nticks is reasonably big, so 1/log(1-1/nticks) is well-approximated by
+ * -nticks.
+ *
+ * To compute log(u), we sample an integer in [1, 64] and divide, then just look
+ * up results in a table.  As a space-compression mechanism, we store these as
+ * uint8_t by dividing the range (255) by the highest-magnitude value the log
+ * can take on, and using that as a multiplier.  We then have to divide by that
+ * multiplier at the end of the computation.
+ *
+ * The values here are computed in src/ticker.py
+ */
+
+const uint8_t ticker_geom_table[1 << TICKER_GEOM_NBITS] = {
+	254, 211, 187, 169, 156, 144, 135, 127,
+	120, 113, 107, 102, 97, 93, 89, 85,
+	81, 77, 74, 71, 68, 65, 62, 60,
+	57, 55, 53, 50, 48, 46, 44, 42,
+	40, 39, 37, 35, 33, 32, 30, 29,
+	27, 26, 24, 23, 21, 20, 19, 18,
+	16, 15, 14, 13, 12, 10, 9, 8,
+	7, 6, 5, 4, 3, 2, 1, 0
+};

src/ticker.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+
+import math
+
+# Must match TICKER_GEOM_NBITS
+lg_table_size = 6
+table_size = 2**lg_table_size
+byte_max = 255
+mul = math.floor(-byte_max/math.log(1 / table_size))
+values = [round(-mul * math.log(i / table_size))
+	for i in range(1, table_size+1)]
+print("mul =", mul)
+print("values:")
+for i in range(table_size // 8):
+	print(", ".join((str(x) for x in values[i*8 : i*8 + 8])))