Linked failure: Add '100 generations' bench test (#1868)

src/test/bench/task-perf-jargon-metal-smoke.rs

@@ -0,0 +1,32 @@
+// Test performance of a task "spawn ladder", in which children task have many
+// many ancestor taskgroups, but with only a few such groups alive at a time.
+// Each child task has to enlist as a descendant in each of its ancestor
+// groups, but that shouldn't have to happen for already-dead groups.
+//
+// The filename is a reference; google it in quotes.
+
+fn child_generation(gens_left: uint) {
+    // This used to be O(n^2) in the number of generations that ever existed.
+    // With this code, only as many generations are alive at a time as tasks
+    // alive at a time, 
+    do task::spawn_supervised {
+        if gens_left & 1 == 1 {
+            task::yield(); // shake things up a bit
+        }
+        if gens_left > 0 {
+            child_generation(gens_left - 1); // recurse
+        }
+    }
+}
+
+fn main(args: ~[~str]) {
+    let args = if os::getenv(~"RUST_BENCH").is_some() {
+        ~[~"", ~"100000"]
+    } else if args.len() <= 1u {
+        ~[~"", ~"100"]
+    } else {
+        copy args
+    };
+
+    child_generation(uint::from_str(args[1]).get());
+}