+static bool
+assign_effective_io_concurrency(int newval, bool doit, GucSource source)
+{
+#ifdef USE_PREFETCH
+ double new_prefetch_pages = 0.0;
+ int i;
+
+ /*----------
+ * The user-visible GUC parameter is the number of drives (spindles),
+ * which we need to translate to a number-of-pages-to-prefetch target.
+ *
+ * The expected number of prefetch pages needed to keep N drives busy is:
+ *
+ * drives | I/O requests
+ * -------+----------------
+ * 1 | 1
+ * 2 | 2/1 + 2/2 = 3
+ * 3 | 3/1 + 3/2 + 3/3 = 5 1/2
+ * 4 | 4/1 + 4/2 + 4/3 + 4/4 = 8 1/3
+ * n | n * H(n)
+ *
+ * This is called the "coupon collector problem" and H(n) is called the
+ * harmonic series. This could be approximated by n * ln(n), but for
+ * reasonable numbers of drives we might as well just compute the series.
+ *
+ * Alternatively we could set the target to the number of pages necessary
+ * so that the expected number of active spindles is some arbitrary
+ * percentage of the total. This sounds the same but is actually slightly
+ * different. The result ends up being ln(1-P)/ln((n-1)/n) where P is
+ * that desired fraction.
+ *
+ * Experimental results show that both of these formulas aren't aggressive
+ * enough, but we don't really have any better proposals.
+ *
+ * Note that if newval = 0 (disabled), we must set target = 0.
+ *----------
+ */
+
+ for (i = 1; i <= newval; i++)
+ new_prefetch_pages += (double) newval / (double) i;
+
+ /* This range check shouldn't fail, but let's be paranoid */
+ if (new_prefetch_pages >= 0.0 && new_prefetch_pages < (double) INT_MAX)
+ {
+ if (doit)
+ target_prefetch_pages = (int) rint(new_prefetch_pages);
+ return true;
+ }
+ else
+ return false;
+#else
+ return true;
+#endif /* USE_PREFETCH */
+}
+