SapMachine SAP#2137: Linux: We should use transparent huge pages in some configurations (SAP#2138)

Linux: Enable UseTransparentHugePages if
- UseLargePages and UseTransparentHugePages are not set explicitly
- transparent huge pages are supported by the system configuration and the huge pages size is not above 2MB
- UseZGC or UseShenandoahGC are not selected
- the Java Heap is not set to 128MB or lower

Author: TheRealMDoerr

src/hotspot/os/linux/os_linux.cpp

@@ -3977,6 +3977,17 @@ void os::Linux::large_page_init() {
   // Query OS information first.
   HugePages::initialize();
 
+  // SapMachine 2025-12-10 Enable UseTransparentHugePages if supported and the huge pages
+  // are not extremely large and no other configuration is selected by flags.
+  // Some GCs have additional requirements, are optimized for ultra-low latency or
+  // have limitations regarding configuration parameters with small heap sizes.
+  if (FLAG_IS_DEFAULT(UseLargePages) && FLAG_IS_DEFAULT(UseTransparentHugePages) &&
+      HugePages::supports_thp() && HugePages::thp_pagesize() <= 2*M &&
+      !UseZGC && !UseShenandoahGC &&
+      (FLAG_IS_DEFAULT(MaxHeapSize) || MaxHeapSize > 128*M)) {
+    _thp_requested = UseTransparentHugePages = true;
+  }
+
   // If THPs are unconditionally enabled (THP mode "always"), khugepaged may attempt to
   // coalesce small pages in thread stacks to huge pages. That costs a lot of memory and
   // is usually unwanted for thread stacks. Therefore we attempt to prevent THP formation in

src/java.base/share/man/java.md

@@ -20,6 +20,8 @@
 # or visit www.oracle.com if you need additional information or have any
 # questions.
 #
+# SapMachine 2025-12-10 Changed description of UseTransparentHugePages which is
+# tested by us.
 
 title: 'JAVA(1) JDK @@VERSION_SHORT@@ | JDK Commands'
 date: @@COPYRIGHT_YEAR@@
@@ -1576,9 +1578,11 @@ These `java` options control the runtime behavior of the Java HotSpot VM.
 
 `-XX:+UseTransparentHugePages`
 :   **Linux only:** Enables the use of large pages that can dynamically grow or
-    shrink. This option is disabled by default. You may encounter performance
-    problems with transparent huge pages as the OS moves other pages around to
-    create huge pages; this option is made available for experimentation.
+    shrink. The VM may enable this option by default in some environments. You
+    may encounter performance problems with transparent huge pages as the OS
+    moves other pages around to create huge pages. You may encounter other
+    performance problems when only small pages are used as the OS needs to
+    maintain a large page table and address translations may become slow.
 
 `-XX:+AllowUserSignalHandlers`
 :   **Non-Windows:** Enables installation of signal handlers by the application. By default,

test/hotspot/jtreg/compiler/codecache/CheckSegmentedCodeCache.java

@@ -231,12 +231,13 @@ public static void main(String[] args) throws Exception {
 
         // Reserved code cache is set, NonNmethod segment size is set, two other segments is automatically
         // adjusted to half of the remaining space
+        // SapMachine 2025-12-10 We support 2MB transparent huge pages, so we need a multiple of it.
         pb = ProcessTools.createLimitedTestJavaProcessBuilder("-XX:+SegmentedCodeCache",
-                                                              "-XX:ReservedCodeCacheSize=100M",
+                                                              "-XX:ReservedCodeCacheSize=102M", // original: 100M
                                                               "-XX:NonNMethodCodeHeapSize=10M",
                                                               "-XX:+PrintFlagsFinal",
                                                               "-version");
-        verifyCodeHeapSize(pb, " ProfiledCodeHeapSize", 47185920);
+        verifyCodeHeapSize(pb, " ProfiledCodeHeapSize", 48234496); // = 46M, original: 47185920
 
         // Reserved code cache is set but NonNmethodCodeHeapSize is not set.
         // It's calculated based on the number of compiler threads

test/hotspot/jtreg/compiler/codecache/cli/codeheapsize/GenericCodeHeapSizeRunner.java

@@ -44,30 +44,34 @@ public void run(CodeCacheCLITestCase.Description testCaseDescription,
                 Long.toString(expectedValues.reserved),
                 String.format("%s should have value %d.",
                         BlobType.All.sizeOptionName, expectedValues.reserved),
-                testCaseDescription.getTestOptions(options));
+                // SapMachine 2025-12-10 We don't get exact matches when rounding to large page sizes.
+                testCaseDescription.getTestOptions(options, "-XX:-UseLargePages"));
 
         CommandLineOptionTest.verifyOptionValueForSameVM(
                 BlobType.NonNMethod.sizeOptionName,
                 Long.toString(expectedValues.nonNmethods),
                 String.format("%s should have value %d.",
                         BlobType.NonNMethod.sizeOptionName,
                         expectedValues.nonNmethods),
-                testCaseDescription.getTestOptions(options));
+                // SapMachine 2025-12-10 We don't get exact matches when rounding to large page sizes.
+                testCaseDescription.getTestOptions(options, "-XX:-UseLargePages"));
 
         CommandLineOptionTest.verifyOptionValueForSameVM(
                 BlobType.MethodNonProfiled.sizeOptionName,
                 Long.toString(expectedValues.nonProfiled),
                 String.format("%s should have value %d.",
                         BlobType.MethodNonProfiled.sizeOptionName,
                         expectedValues.nonProfiled),
-                testCaseDescription.getTestOptions(options));
+                // SapMachine 2025-12-10 We don't get exact matches when rounding to large page sizes.
+                testCaseDescription.getTestOptions(options, "-XX:-UseLargePages"));
 
         CommandLineOptionTest.verifyOptionValueForSameVM(
                 BlobType.MethodProfiled.sizeOptionName,
                 Long.toString(expectedValues.profiled),
                 String.format("%s should have value %d.",
                         BlobType.MethodProfiled.sizeOptionName,
                         expectedValues.profiled),
-                testCaseDescription.getTestOptions(options));
+                // SapMachine 2025-12-10 We don't get exact matches when rounding to large page sizes.
+                testCaseDescription.getTestOptions(options, "-XX:-UseLargePages"));
     }
 }

test/hotspot/jtreg/runtime/os/TestHugePageDecisionsAtVMStartup.java

@@ -22,14 +22,15 @@
  * questions.
  */
 
+// SapMachine 2025-12-10 We use UseTransparentHugePages in some configurations.
 /*
  * @test id=Default
  * @summary Test JVM large page setup (default options)
  * @library /test/lib
  * @requires os.family == "linux"
  * @modules java.base/jdk.internal.misc
  *          java.management
- * @run driver TestHugePageDecisionsAtVMStartup
+ * @run driver TestHugePageDecisionsAtVMStartup -XX:-UseTransparentHugePages
  */
 
 /*