Don't Allocate Redundant Pages in BigArrays (#60201)

The oversize algorithm was allocating more pages than necessary to accommodate `minTargetSize`.
An example would be that a 16k page size and 15k `minTargetSize` would result in a new size of 32k (2 pages).
The difference between the minimum number of necessary pages and the estimated size then keeps growing as sizes increase.

I don't think there is much value in preemptively allocating pages by over-sizing aggressively since the behavior of
the system is quite different from that of a single array where over-sizing avoids copying
once the minimum target size is more than a single page.

Relates #60173 which lead me to this when `BytesStreamOutput` would allocate a large number of never used
pages during serialization of repository metadata.

server/src/main/java/org/elasticsearch/common/util/BigArrays.java

@@ -58,15 +58,10 @@ public class BigArrays {
 
         long newSize;
         if (minTargetSize < pageSize) {
-            newSize = ArrayUtil.oversize((int)minTargetSize, bytesPerElement);
+            newSize = Math.min(ArrayUtil.oversize((int) minTargetSize, bytesPerElement), pageSize);
         } else {
-            newSize = minTargetSize + (minTargetSize >>> 3);
-        }
-
-        if (newSize > pageSize) {
-            // round to a multiple of pageSize
-            newSize = newSize - (newSize % pageSize) + pageSize;
-            assert newSize % pageSize == 0;
+            final long pages = (minTargetSize + pageSize - 1) / pageSize; // ceil(minTargetSize/pageSize)
+            newSize = pages * pageSize;
         }
 
         return newSize;
@@ -811,4 +806,3 @@ public class BigArrays {
         return resize(array, newSize);
     }
 }
-

server/src/test/java/org/elasticsearch/common/util/BigArraysTests.java

@@ -38,7 +38,9 @@ import java.util.List;
 import java.util.function.Function;
 
 import static org.elasticsearch.indices.breaker.HierarchyCircuitBreakerService.REQUEST_CIRCUIT_BREAKER_LIMIT_SETTING;
+import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThanOrEqualTo;
+import static org.hamcrest.Matchers.lessThan;
 
 public class BigArraysTests extends ESTestCase {
 
@@ -389,6 +391,17 @@ public class BigArraysTests extends ESTestCase {
         }
     }
 
+    public void testOverSizeUsesMinPageCount() {
+        final int pageSize = 1 << (randomIntBetween(2, 16));
+        final int minSize = randomIntBetween(1, pageSize) * randomIntBetween(1, 100);
+        final long size = BigArrays.overSize(minSize, pageSize, 1);
+        assertThat(size, greaterThanOrEqualTo((long)minSize));
+        if (size >= pageSize) {
+            assertThat(size + " is a multiple of " + pageSize, size % pageSize, equalTo(0L));
+        }
+        assertThat(size - minSize, lessThan((long) pageSize));
+    }
+
     private List<BigArraysHelper> bigArrayCreators(final long maxSize, final boolean withBreaking) {
         final BigArrays byteBigArrays = newBigArraysInstance(maxSize, withBreaking);
         BigArraysHelper byteHelper = new BigArraysHelper(byteBigArrays,