nodestack: implement expiration sloppiness parameter

Now, a previously-explored, repeated future value will only generate a
new future-value/expiration pair if the expiration is higher than the
previous expiration + sloppiness. This prevents executions where we
continue to step up the future value expiration by a small increment,
creating the same execution.

This also rewrites the add_future_value() logic just a bit so that it is
a little bit more clear.

diff --git a/nodestack.cc b/nodestack.cc
index f73d4cf4a154e264ed93a32f250c6a0f6dfaa11a..b4f75e0336a3afdc3978b7ecb086e459423df455 100644 (file)
--- a/nodestack.cc
+++ b/nodestack.cc
@@ -193,30 +193,33 @@ bool Node::misc_empty() {
 
 /**
  * Adds a value from a weakly ordered future write to backtrack to. This
- * operation may "fail" if the future value is already set (with a later
- * expiration), or if the futurevalues set has reached its maximum.
+ * operation may "fail" if the future value has already been run (within some
+ * sloppiness window of this expiration), or if the futurevalues set has
+ * reached its maximum.
  * @see model_params.maxfuturevalues
  *
  * @param value is the value to backtrack to.
  * @return True if the future value was successully added; false otherwise
  */
 bool Node::add_future_value(uint64_t value, modelclock_t expiration) {
-       int suitableindex=-1;
+       int idx = -1; /* Highest index where value is found */
        for (unsigned int i = 0; i < future_values.size(); i++) {
                if (future_values[i].value == value) {
-                       if (future_values[i].expiration>=expiration)
+                       if (expiration <= future_values[i].expiration)
                                return false;
-                       if (future_index < ((int) i)) {
-                               suitableindex=i;
-                       }
+                       idx = i;
                }
        }
-
-       if (suitableindex!=-1) {
-               future_values[suitableindex].expiration=expiration;
+       if (idx > future_index) {
+               /* Future value hasn't been explored; update expiration */
+               future_values[idx].expiration = expiration;
                return true;
+       } else if (idx >= 0 && expiration <= future_values[idx].expiration + model->params.expireslop) {
+               /* Future value has been explored and is within the "sloppy" window */
+               return false;
        }
 
+       /* Limit the size of the future-values set */
        if (model->params.maxfuturevalues > 0 &&
                        (int)future_values.size() >= model->params.maxfuturevalues)
                return false;