Arena to inline 2KB of data in it.

Summary:
In order to use arena to a use case that the total allocation size might be small (LogBuffer is already such a case), inline 1KB of data in it, so that it can be mostly in stack or inline in another class.

If always inlining 2KB is a concern, I could make it a template to determine what to inline. However, dependents need to changes. Doesn't go with it for now

Test Plan: make all check.

Reviewers: haobo, igor, yhchiang, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18609

db/db_test.cc

@@ -2365,9 +2365,9 @@ TEST(DBTest, NumImmutableMemTable) {
     ASSERT_EQ(num, "0");
     ASSERT_TRUE(dbfull()->GetProperty(
         handles_[1], "rocksdb.cur-size-active-mem-table", &num));
-    // "208" is the size of the metadata of an empty skiplist, this would
+    // "200" is the size of the metadata of an empty skiplist, this would
     // break if we change the default skiplist implementation
-    ASSERT_EQ(num, "208");
+    ASSERT_EQ(num, "200");
     SetPerfLevel(kDisable);
   } while (ChangeCompactOptions());
 }

util/arena.cc

@@ -34,6 +34,10 @@ size_t OptimizeBlockSize(size_t block_size) {
 Arena::Arena(size_t block_size) : kBlockSize(OptimizeBlockSize(block_size)) {
   assert(kBlockSize >= kMinBlockSize && kBlockSize <= kMaxBlockSize &&
          kBlockSize % kAlignUnit == 0);
+  alloc_bytes_remaining_ = sizeof(inline_block_);
+  blocks_memory_ += alloc_bytes_remaining_;
+  aligned_alloc_ptr_ = inline_block_;
+  unaligned_alloc_ptr_ = inline_block_ + alloc_bytes_remaining_;
 }
 
 Arena::~Arena() {
@@ -71,17 +75,17 @@ char* Arena::AllocateFallback(size_t bytes, bool aligned) {
   }
 }
 
-char* Arena::AllocateAligned(size_t bytes, size_t huge_page_tlb_size,
+char* Arena::AllocateAligned(size_t bytes, size_t huage_page_size,
                              Logger* logger) {
   assert((kAlignUnit & (kAlignUnit - 1)) ==
          0);  // Pointer size should be a power of 2
 
 #ifdef MAP_HUGETLB
-  if (huge_page_tlb_size > 0 && bytes > 0) {
+  if (huage_page_size > 0 && bytes > 0) {
     // Allocate from a huge page TBL table.
     assert(logger != nullptr);  // logger need to be passed in.
     size_t reserved_size =
-        ((bytes - 1U) / huge_page_tlb_size + 1U) * huge_page_tlb_size;
+        ((bytes - 1U) / huage_page_size + 1U) * huage_page_size;
     assert(reserved_size >= bytes);
     void* addr = mmap(nullptr, reserved_size, (PROT_READ | PROT_WRITE),
                       (MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB), 0, 0);

util/arena.h

@@ -28,6 +28,7 @@ class Arena {
   Arena(const Arena&) = delete;
   void operator=(const Arena&) = delete;
 
+  static const size_t kInlineSize = 2048;
   static const size_t kMinBlockSize;
   static const size_t kMaxBlockSize;
 
@@ -36,18 +37,19 @@ class Arena {
 
   char* Allocate(size_t bytes);
 
-  // huge_page_tlb_size: if >0, allocate bytes from huge page TLB and the size
-  // of the huge page TLB. Bytes will be rounded up to multiple and 2MB and
-  // allocate huge pages through mmap anonymous option with huge page on.
-  // The extra  space allocated will be wasted. To enable it, need to reserve
-  // huge pages for it to be allocated, like:
+  // huge_page_size: if >0, will try to allocate from huage page TLB.
+  // The argument will be the size of the page size for huge page TLB. Bytes
+  // will be rounded up to multiple of the page size to allocate through mmap
+  // anonymous option with huge page on. The extra  space allocated will be
+  // wasted. If allocation fails, will fall back to normal case. To enable it,
+  // need to reserve huge pages for it to be allocated, like:
   //     sysctl -w vm.nr_hugepages=20
   // See linux doc Documentation/vm/hugetlbpage.txt for details.
   // huge page allocation can fail. In this case it will fail back to
   // normal cases. The messages will be logged to logger. So when calling with
   // huge_page_tlb_size > 0, we highly recommend a logger is passed in.
   // Otherwise, the error message will be printed out to stderr directly.
-  char* AllocateAligned(size_t bytes, size_t huge_page_tlb_size = 0,
+  char* AllocateAligned(size_t bytes, size_t huge_page_size = 0,
                         Logger* logger = nullptr);
 
   // Returns an estimate of the total memory usage of data allocated
@@ -69,6 +71,7 @@ class Arena {
   size_t BlockSize() const { return kBlockSize; }
 
  private:
+  char inline_block_[kInlineSize];
   // Number of bytes allocated in one block
   const size_t kBlockSize;
   // Array of new[] allocated memory blocks

util/arena_test.cc

@@ -31,9 +31,11 @@ TEST(ArenaTest, MemoryAllocatedBytes) {
   for (int i = 0; i < N; i++) {
     arena.Allocate(req_sz);
   }
-  expected_memory_allocated = req_sz * N;
+  expected_memory_allocated = req_sz * N + Arena::kInlineSize;
   ASSERT_EQ(arena.MemoryAllocatedBytes(), expected_memory_allocated);
 
+  arena.Allocate(Arena::kInlineSize - 1);
+
   // requested size < quarter of a block:
   //   allocate a block with the default size, then try to use unused part
   //   of the block. So one new block will be allocated for the first
@@ -64,12 +66,19 @@ TEST(ArenaTest, ApproximateMemoryUsageTest) {
   Arena arena(kBlockSize);
   ASSERT_EQ(kZero, arena.ApproximateMemoryUsage());
 
+  // allocate inline bytes
+  arena.AllocateAligned(8);
+  arena.AllocateAligned(Arena::kInlineSize / 2 - 16);
+  arena.AllocateAligned(Arena::kInlineSize / 2);
+  ASSERT_EQ(arena.ApproximateMemoryUsage(), Arena::kInlineSize - 8);
+  ASSERT_EQ(arena.MemoryAllocatedBytes(), Arena::kInlineSize);
+
   auto num_blocks = kBlockSize / kEntrySize;
 
   // first allocation
   arena.AllocateAligned(kEntrySize);
   auto mem_usage = arena.MemoryAllocatedBytes();
-  ASSERT_EQ(mem_usage, kBlockSize);
+  ASSERT_EQ(mem_usage, kBlockSize + Arena::kInlineSize);
   auto usage = arena.ApproximateMemoryUsage();
   ASSERT_LT(usage, mem_usage);
   for (size_t i = 1; i < num_blocks; ++i) {