[GR-62544] Reinstate size-based full collection decision in adaptive policy.

PullRequest: graal/20152

Author: peter-hofer

substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/AdaptiveCollectionPolicy.java

@@ -26,7 +26,6 @@
 
 import static com.oracle.svm.core.genscavenge.CollectionPolicy.shouldCollectYoungGenSeparately;
 
-import jdk.graal.compiler.word.Word;
 import org.graalvm.word.UnsignedWord;
 
 import com.oracle.svm.core.Uninterruptible;
@@ -35,6 +34,8 @@
 import com.oracle.svm.core.util.TimeUtils;
 import com.oracle.svm.core.util.UnsignedUtils;
 
+import jdk.graal.compiler.word.Word;
+
 /**
  * A garbage collection policy that balances throughput and memory footprint.
  *
@@ -163,7 +164,8 @@ public String getName() {
     public boolean shouldCollectCompletely(boolean followingIncrementalCollection) { // should_attempt_scavenge
         guaranteeSizeParametersInitialized();
 
-        if (!followingIncrementalCollection && shouldCollectYoungGenSeparately(!SerialGCOptions.useCompactingOldGen())) {
+        boolean collectYoungSeparately = shouldCollectYoungGenSeparately(!SerialGCOptions.useCompactingOldGen());
+        if (!followingIncrementalCollection && collectYoungSeparately) {
             /*
              * With a copying collector, default to always doing an incremental collection first
              * because we expect most of the objects in the young generation to be garbage, and we
@@ -181,6 +183,12 @@ public boolean shouldCollectCompletely(boolean followingIncrementalCollection) {
              */
             return true;
         }
+
+        if (!collectYoungSeparately && followingIncrementalCollection) {
+            // Don't override the earlier decision to not do a full GC below (and prolong the pause)
+            return false;
+        }
+
         if (minorCountSinceMajorCollection * avgMinorPause.getAverage() >= CONSECUTIVE_MINOR_TO_MAJOR_COLLECTION_PAUSE_TIME_RATIO * avgMajorPause.getPaddedAverage()) {
             /*
              * When we do many incremental collections in a row because they reclaim sufficient
@@ -190,7 +198,17 @@ public boolean shouldCollectCompletely(boolean followingIncrementalCollection) {
             return true;
         }
 
-        return false;
+        UnsignedWord youngUsed = HeapImpl.getHeapImpl().getYoungGeneration().getChunkBytes();
+        UnsignedWord oldUsed = HeapImpl.getHeapImpl().getOldGeneration().getChunkBytes();
+
+        /*
+         * If the remaining free space in the old generation is less than what is expected to be
+         * needed by the next collection, do a full collection now.
+         */
+        UnsignedWord averagePromoted = UnsignedUtils.fromDouble(avgPromoted.getPaddedAverage());
+        UnsignedWord promotionEstimate = UnsignedUtils.min(averagePromoted, youngUsed);
+        UnsignedWord oldFree = oldSize.subtract(oldUsed);
+        return promotionEstimate.aboveThan(oldFree);
     }
 
     private void updateAverages(boolean isSurvivorOverflow, UnsignedWord survivedChunkBytes, UnsignedWord promotedChunkBytes) {