6973570: OrderAccess::storestore() scales poorly on multi-socket x64 and...

6973570: OrderAccess::storestore() scales poorly on multi-socket x64 and sparc: cache-line ping-ponging
Summary: volatile store to static variable removed in favour of a volatile store to stack to avoid excessive cache coherency traffic; verified that the volatile store is not elided by any of our current compilers.
Reviewed-by: dholmes, dice, jcoomes, kvn

src/os_cpu/linux_sparc/vm/orderAccess_linux_sparc.inline.hpp

 /*
- * Copyright (c) 2003, 2008, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -36,8 +36,8 @@ inline void OrderAccess::acquire() {
 }
 
 inline void OrderAccess::release() {
-  jint* dummy = (jint*)&dummy;
-  __asm__ volatile("stw %%g0, [%0]" : : "r" (dummy) : "memory");
+  jint* local_dummy = (jint*)&local_dummy;
+  __asm__ volatile("stw %%g0, [%0]" : : "r" (local_dummy) : "memory");
 }
 
 inline void OrderAccess::fence() {

src/os_cpu/linux_x86/vm/orderAccess_linux_x86.inline.hpp

 /*
- * Copyright (c) 2003, 2009, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -30,16 +30,18 @@ inline void OrderAccess::loadstore()  { acquire(); }
 inline void OrderAccess::storeload()  { fence(); }
 
 inline void OrderAccess::acquire() {
-  volatile intptr_t dummy;
+  volatile intptr_t local_dummy;
 #ifdef AMD64
-  __asm__ volatile ("movq 0(%%rsp), %0" : "=r" (dummy) : : "memory");
+  __asm__ volatile ("movq 0(%%rsp), %0" : "=r" (local_dummy) : : "memory");
 #else
-  __asm__ volatile ("movl 0(%%esp),%0" : "=r" (dummy) : : "memory");
+  __asm__ volatile ("movl 0(%%esp),%0" : "=r" (local_dummy) : : "memory");
 #endif // AMD64
 }
 
 inline void OrderAccess::release() {
-  dummy = 0;
+  // Avoid hitting the same cache-line from
+  // different threads.
+  volatile jint local_dummy = 0;
 }
 
 inline void OrderAccess::fence() {

src/os_cpu/solaris_sparc/vm/orderAccess_solaris_sparc.inline.hpp

 /*
- * Copyright (c) 2003, 2009, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -42,8 +42,8 @@ inline void OrderAccess::acquire() {
 }
 
 inline void OrderAccess::release() {
-  jint* dummy = (jint*)&dummy;
-  __asm__ volatile("stw %%g0, [%0]" : : "r" (dummy) : "memory");
+  jint* local_dummy = (jint*)&local_dummy;
+  __asm__ volatile("stw %%g0, [%0]" : : "r" (local_dummy) : "memory");
 }
 
 inline void OrderAccess::fence() {
@@ -57,7 +57,9 @@ inline void OrderAccess::acquire() {
 }
 
 inline void OrderAccess::release() {
-  dummy = 0;
+  // Avoid hitting the same cache-line from
+  // different threads.
+  volatile jint local_dummy = 0;
 }
 
 inline void OrderAccess::fence() {

src/os_cpu/solaris_x86/vm/orderAccess_solaris_x86.inline.hpp

 /*
- * Copyright (c) 2003, 2009, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -40,7 +40,9 @@ inline void OrderAccess::acquire() {
 }
 
 inline void OrderAccess::release() {
-  dummy = 0;
+  // Avoid hitting the same cache-line from
+  // different threads.
+  volatile jint local_dummy = 0;
 }
 
 inline void OrderAccess::fence() {
@@ -53,11 +55,11 @@ inline void OrderAccess::fence() {
 
 extern "C" {
   inline void _OrderAccess_acquire() {
-    volatile intptr_t dummy;
+    volatile intptr_t local_dummy;
 #ifdef AMD64
-    __asm__ volatile ("movq 0(%%rsp), %0" : "=r" (dummy) : : "memory");
+    __asm__ volatile ("movq 0(%%rsp), %0" : "=r" (local_dummy) : : "memory");
 #else
-    __asm__ volatile ("movl 0(%%esp),%0" : "=r" (dummy) : : "memory");
+    __asm__ volatile ("movl 0(%%esp),%0" : "=r" (local_dummy) : : "memory");
 #endif // AMD64
   }
   inline void _OrderAccess_fence() {

src/os_cpu/windows_x86/vm/orderAccess_windows_x86.inline.hpp

 /*
- * Copyright (c) 2003, 2009, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -41,7 +41,7 @@ inline void OrderAccess::acquire() {
 
 inline void OrderAccess::release() {
   // A volatile store has release semantics.
-  dummy = 0;
+  volatile jint local_dummy = 0;
 }
 
 inline void OrderAccess::fence() {

src/share/vm/runtime/orderAccess.cpp

 /*
- * Copyright (c) 2003, 2009, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -25,8 +25,6 @@
 # include "incls/_precompiled.incl"
 # include "incls/_orderAccess.cpp.incl"
 
-volatile intptr_t OrderAccess::dummy = 0;
-
 void OrderAccess::StubRoutines_fence() {
   // Use a stub if it exists.  It may not exist during bootstrap so do
   // nothing in that case but assert if no fence code exists after threads have been created

src/share/vm/runtime/orderAccess.hpp

 /*
- * Copyright (c) 2003, 2009, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -166,6 +166,12 @@
 // and release must include a sequence point, usually via a volatile memory
 // access.  Other ways to guarantee a sequence point are, e.g., use of
 // indirect calls and linux's __asm__ volatile.
+// Note: as of 6973570, we have replaced the originally static "dummy" field
+// (see above) by a volatile store to the stack. All of the versions of the
+// compilers that we currently use (SunStudio, gcc and VC++) respect the
+// semantics of volatile here. If you build HotSpot using other
+// compilers, you may need to verify that no compiler reordering occurs
+// across the sequence point respresented by the volatile access.
 //
 //
 //                os::is_MP Considered Redundant
@@ -297,10 +303,6 @@ class OrderAccess : AllStatic {
   static void     release_store_ptr_fence(volatile intptr_t* p, intptr_t v);
   static void     release_store_ptr_fence(volatile void*     p, void*    v);
 
-  // In order to force a memory access, implementations may
-  // need a volatile externally visible dummy variable.
-  static volatile intptr_t dummy;
-
  private:
   // This is a helper that invokes the StubRoutines::fence_entry()
   // routine if it exists, It should only be used by platforms that