IOBuf combine allocations only within size classes

Summary: IOBuf allocate SharedInfo in the same allocation only if we do not cross into the next class size

Reviewed By: jrahman

Differential Revision: D27353914

fbshipit-source-id: 65be3641d7eaa0588bf26101ce02668fb827eee9

folly/io/IOBuf.cpp

@@ -276,6 +276,21 @@ unique_ptr<IOBuf> IOBuf::create(std::size_t capacity) {
   if (capacity <= kDefaultCombinedBufSize) {
     return createCombined(capacity);
   }
+
+  // if we have nallocx, we want to allocate the capacity and the overhead in
+  // a single allocation only if we do not cross into the next allocation class
+  // for some buffer sizes, this can use about 25% extra memory
+  if (canNallocx()) {
+    auto mallocSize = goodMallocSize(capacity);
+    // round capacity to a multiple of 8
+    size_t minSize = ((capacity + 7) & ~7) + sizeof(SharedInfo);
+    // if we do not have space for the overhead, allocate the mem separateley
+    if (mallocSize < minSize) {
+      auto* buf = checkedMalloc(mallocSize);
+      return takeOwnership(buf, mallocSize, static_cast<size_t>(0));
+    }
+  }
+
   return createSeparate(capacity);
 }
 

folly/io/test/IOBufTest.cpp

@@ -73,10 +73,8 @@ TEST(IOBuf, Simple) {
 }
 
 void testAllocSize(uint32_t requestedCapacity) {
-  auto expectedSize = IOBuf::goodSize(requestedCapacity);
   unique_ptr<IOBuf> iobuf(IOBuf::create(requestedCapacity));
   EXPECT_GE(iobuf->capacity(), requestedCapacity);
-  EXPECT_EQ(iobuf->capacity(), expectedSize);
 }
 
 TEST(IOBuf, AllocSizes) {