Rounds up VirtualAlloc calls on windows to dwAllocationGranularity to prevent fragmentation

third_party/tcmalloc/port.cc

 /* Copyright (c) 2007, Google Inc.
  * All rights reserved.
  * 
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  * 
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 53 matching lines @@
   va_end(ap);
   return r;
 }
 #endif
 
 int getpagesize() {
   static int pagesize = 0;
   if (pagesize == 0) {
     SYSTEM_INFO system_info;
     GetSystemInfo(&system_info);
-    pagesize = system_info.dwPageSize;
+    pagesize = std::max(system_info.dwPageSize,
+                        system_info.dwAllocationGranularity);
   }
   return pagesize;
 }
 
 extern "C" PERFTOOLS_DLL_DECL void* __sbrk(ptrdiff_t increment) {
   LOG(FATAL, "Windows doesn't implement sbrk!\n");
   return NULL;
 }
 
 // -----------------------------------------------------------------------
@@ skipping 96 matching lines @@
 
 // -----------------------------------------------------------------------
 // These functions replace system-alloc.cc
 
 static SpinLock alloc_lock(SpinLock::LINKER_INITIALIZED);
 
 // This is mostly like MmapSysAllocator::Alloc, except it does these weird
 // munmap's in the middle of the page, which is forbidden in windows.
 extern void* TCMalloc_SystemAlloc(size_t size, size_t *actual_size,
                                   size_t alignment) {
-  // Safest is to make actual_size same as input-size.
-  if (actual_size) {
-    *actual_size = size;
-  }
-
   SpinLockHolder sh(&alloc_lock);
   // Align on the pagesize boundary
   const int pagesize = getpagesize();
   if (alignment < pagesize) alignment = pagesize;
   size = ((size + alignment - 1) / alignment) * alignment;
 
-  // Ask for extra memory if alignment > pagesize
-  size_t extra = 0;
-  if (alignment > pagesize) {
-    extra = alignment - pagesize;
+  // Safest is to make actual_size same as input-size.

[jar (doing other things), 2009/09/24 23:24:34]

The unexplained "safest" comment could be elaborated to:

Report the total number of bytes the OS delivered, rather than just |size| so
that spans can include the full amount, and not be artificially separated,
precluding coalescing.

+  // TODO(antonm): proper processing of alignments
+  // in actual_size and decommitting.

[jar (doing other things), 2009/09/24 23:24:34]

I don't understand this TODO comment.  Can you clarify?  Perhaps it should be
removed now that we have the assertions?

+  if (actual_size) {
+    *actual_size = size;
   }
 
-  void* result = VirtualAlloc(0, size + extra,
+  // We currently do not support alignments larger than the pagesize or
+  // alignments that are not multiples of the pagesize after being floored.
+  // If this ability is needed it can be done by the caller (assuming it knows
+  // the page size).
+  assert(alignment <= pagesize);
+
+  void* result = VirtualAlloc(0, size,
                               MEM_COMMIT|MEM_RESERVE, PAGE_READWRITE);
   if (result == NULL)
     return NULL;
 
-  // Adjust the return memory so it is aligned
-  uintptr_t ptr = reinterpret_cast<uintptr_t>(result);
-  size_t adjust = 0;
-  if ((ptr & (alignment - 1)) != 0) {
-    adjust = alignment - (ptr & (alignment - 1));
-  }
+  // If the result is not aligned memory fragmentation will result which can
+  // lead to pathological memory use.
+  assert((reinterpret_cast<uintptr_t>(result) & (alignment - 1)) == 0);
 
-  ptr += adjust;
-  return reinterpret_cast<void*>(ptr);
+  return result;
 }
 
 void TCMalloc_SystemRelease(void* start, size_t length) {
   if (VirtualFree(start, length, MEM_DECOMMIT))
     return;
 
   // The decommit may fail if the memory region consists of allocations
   // from more than one call to VirtualAlloc.  In this case, fall back to
   // using VirtualQuery to retrieve the allocation boundaries and decommit
   // them each individually.
@@ skipping 60 matching lines @@
       if ((strlen(fname) >= prefix_length) &&
           (memcmp(fname, prefix, prefix_length) == 0)) {
         RAW_VLOG(0, "Removing old heap profile %s\n", fname);
         // TODO(csilvers): we really need to unlink dirname + fname
         _unlink(fname);
       }
     } while (FindNextFileA(hFind, &found) != FALSE);  // A is for Ansi
     FindClose(hFind);
   }
 }