Truncate app container heap size to 4 significant bits (when hosted)

2 files changed, 37 insertions, 3 deletions

diff --git a/config-model/src/main/java/com/yahoo/vespa/model/container/ApplicationContainerCluster.java b/config-model/src/main/java/com/yahoo/vespa/model/container/ApplicationContainerCluster.java
index 531dc8f0fcf..cc570d47907 100644
--- a/config-model/src/main/java/com/yahoo/vespa/model/container/ApplicationContainerCluster.java
+++ b/config-model/src/main/java/com/yahoo/vespa/model/container/ApplicationContainerCluster.java
@@ -315,16 +315,25 @@ public final class ApplicationContainerCluster extends ContainerCluster<Applicat
     public void getConfig(QrStartConfig.Builder builder) {
         super.getConfig(builder);
         var memoryPct = getMemoryPercentage().orElse(null);
-        int heapsize = memoryPct != null && memoryPct.asAbsoluteGb().isPresent()
-                       ? (int) (memoryPct.asAbsoluteGb().getAsDouble() * 1024) : 1536;
+        int heapsize = truncateTo4SignificantBits(memoryPct != null && memoryPct.asAbsoluteGb().isPresent()
+                                                  ? (int) (memoryPct.asAbsoluteGb().getAsDouble() * 1024) : 1536);
         builder.jvm.verbosegc(true)
                 .availableProcessors(0)
                 .compressedClassSpaceSize(0)
-                .minHeapsize(heapsize)
+                .minHeapsize(heapsize) // These cause restarts when changed, so we try to keep them stable.
                 .heapsize(heapsize);
         if (memoryPct != null) builder.jvm.heapSizeAsPercentageOfPhysicalMemory(memoryPct.ofContainerAvailable());
     }
 
+    static int truncateTo4SignificantBits(int i) {
+        if (i == Integer.MIN_VALUE) return i;
+        if (i < 0) return -truncateTo4SignificantBits(-i);
+        if (i <= 16) return i;
+        int mask = Integer.highestOneBit(i);
+        mask += mask - (mask >> 3);
+        return i & mask;
+    }
+
     @Override
     public void getConfig(ZookeeperServerConfig.Builder builder) {
         if (getParent() instanceof ConfigserverCluster) return; // Produces its own config
diff --git a/config-model/src/test/java/com/yahoo/vespa/model/container/ContainerClusterTest.java b/config-model/src/test/java/com/yahoo/vespa/model/container/ContainerClusterTest.java
index 9cf34b55ebf..93362e144a6 100644
--- a/config-model/src/test/java/com/yahoo/vespa/model/container/ContainerClusterTest.java
+++ b/config-model/src/test/java/com/yahoo/vespa/model/container/ContainerClusterTest.java
@@ -49,6 +49,7 @@ import static com.yahoo.config.model.api.ApplicationClusterEndpoint.Scope.zone;
 import static com.yahoo.config.provision.SystemName.main;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 /**
@@ -498,4 +499,28 @@ public class ContainerClusterTest {
                 (extra.isEmpty() ? "" : " " + extra);
     }
 
+    @Test
+    void testTruncationTo4Bits() {
+        assertEquals(0, ApplicationContainerCluster.truncateTo4SignificantBits(0));
+        assertEquals(1, ApplicationContainerCluster.truncateTo4SignificantBits(1));
+        assertEquals(15, ApplicationContainerCluster.truncateTo4SignificantBits(15));
+        assertEquals(16, ApplicationContainerCluster.truncateTo4SignificantBits(16));
+        assertEquals(16, ApplicationContainerCluster.truncateTo4SignificantBits(17));
+        assertEquals(18, ApplicationContainerCluster.truncateTo4SignificantBits(18));
+        assertEquals(18, ApplicationContainerCluster.truncateTo4SignificantBits(19));
+        assertEquals(30, ApplicationContainerCluster.truncateTo4SignificantBits(30));
+        assertEquals(30, ApplicationContainerCluster.truncateTo4SignificantBits(31));
+        assertEquals(32, ApplicationContainerCluster.truncateTo4SignificantBits(32));
+        assertEquals(32, ApplicationContainerCluster.truncateTo4SignificantBits(33));
+        assertEquals(32, ApplicationContainerCluster.truncateTo4SignificantBits(34));
+        assertEquals(32, ApplicationContainerCluster.truncateTo4SignificantBits(35));
+        assertEquals(36, ApplicationContainerCluster.truncateTo4SignificantBits(36));
+        assertEquals(0x78000000, ApplicationContainerCluster.truncateTo4SignificantBits(0x78000000));
+        assertEquals(0x78000000, ApplicationContainerCluster.truncateTo4SignificantBits(0x7fffffff));
+        assertEquals(0x80000000, ApplicationContainerCluster.truncateTo4SignificantBits(0x80000000));
+        assertEquals(0b10001000000000000000000000000000, ApplicationContainerCluster.truncateTo4SignificantBits(0b10000000000000000000000000000001));
+        assertEquals(0b11000100000000000000000000000000, ApplicationContainerCluster.truncateTo4SignificantBits(0b11000000000000000000000000000001));
+        assertEquals(0b11111111111111111111111111100010, ApplicationContainerCluster.truncateTo4SignificantBits(0b11111111111111111111111111100001));
+    }
+
 }