Use live memory to estimate collection pages in mem balancer (#1050)

This PR fixes #918. We
misinterpreted 'bytes collected during collection' in the paper, and
used reclaimed pages for it. It actually means 'bytes that are traversed
during collection'. We now use live pages to estimate the value.

src/util/heap/gc_trigger.rs

@@ -208,7 +208,7 @@ struct MemBalancerStats {
     allocation_pages: f64,
     /// Allocation duration in secs
     allocation_time: f64,
-    /// Collected memory in pages
+    /// Collected memory in pages (memory traversed during collection)
     collection_pages: f64,
     /// Collection duration in secs
     collection_time: f64,
@@ -289,9 +289,8 @@ impl MemBalancerStats {
     ) -> bool {
         if !plan.is_current_gc_nursery() {
             self.gc_end_live_pages = plan.get_mature_reserved_pages();
-            self.collection_pages = self
-                .gc_release_live_pages
-                .saturating_sub(self.gc_end_live_pages) as f64;
+            // Use live pages as an estimate for pages traversed during GC
+            self.collection_pages = self.gc_end_live_pages as f64;
             trace!(
                 "collected pages = mature live at gc end {} - mature live at gc release {} = {}",
                 self.gc_release_live_pages,
@@ -327,9 +326,8 @@ impl MemBalancerStats {
     fn non_generational_mem_stats_on_gc_end<VM: VMBinding>(&mut self, mmtk: &'static MMTK<VM>) {
         self.gc_end_live_pages = mmtk.get_plan().get_reserved_pages();
         trace!("live pages = {}", self.gc_end_live_pages);
-        self.collection_pages = self
-            .gc_release_live_pages
-            .saturating_sub(self.gc_end_live_pages) as f64;
+        // Use live pages as an estimate for pages traversed during GC
+        self.collection_pages = self.gc_end_live_pages as f64;
         trace!(
             "collected pages = live at gc end {} - live at gc release {} = {}",
             self.gc_release_live_pages,