base/trace_event/heap_profiler_allocation_register.cc

// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/trace_event/heap_profiler_allocation_register.h"

#include "base/trace_event/trace_event_memory_overhead.h"

namespace base {
namespace trace_event {

AllocationRegister::AllocationRegister()
    : AllocationRegister(kNumBuckets * kNumCellsPerBucket) {}

AllocationRegister::AllocationRegister(uint32_t num_cells)
    // Reserve enough address space to store |num_cells_| entries if necessary,
    // with a guard page after it to crash the program when attempting to store
    // more entries.
    : num_cells_(num_cells),
      cells_(static_cast<Cell*>(AllocateVirtualMemory(num_cells_ *
                                                      sizeof(Cell)))),
      buckets_(static_cast<CellIndex*>(
          AllocateVirtualMemory(kNumBuckets * sizeof(CellIndex)))),

      // The free list is empty. The first unused cell is cell 1, because index
      // 0 is used as list terminator.
      free_list_(0),
      next_unused_cell_(1) {}

AllocationRegister::~AllocationRegister() {
  FreeVirtualMemory(buckets_, kNumBuckets * sizeof(CellIndex));
  FreeVirtualMemory(cells_, num_cells_ * sizeof(Cell));
}

void AllocationRegister::Insert(void* address,
                                size_t size,
                                AllocationContext context) {
  DCHECK(address != nullptr);
  if (size == 0)
    return;

  CellIndex* idx_ptr = Lookup(address);

  // If the index is 0, the address is not yet present, so insert it.
  if (*idx_ptr == 0) {
    *idx_ptr = GetFreeCell();

    // The address stored in a cell is const as long as it is exposed (via the
    // iterators or |Get|), but because cells are re-used, a const cast is
    // required to set it on insert and remove.
    void* const& allocation_address = cells_[*idx_ptr].allocation.address;
    const_cast<void*&>(allocation_address) = address;
    cells_[*idx_ptr].next = 0;
  }

  cells_[*idx_ptr].allocation.size = size;
  cells_[*idx_ptr].allocation.context = context;
}

void AllocationRegister::Remove(void* address) {
  // Get a pointer to the index of the cell that stores |address|. The index can
  // be an element of |buckets_| or the |next| member of a cell.
  CellIndex* idx_ptr = Lookup(address);
  CellIndex freed_idx = *idx_ptr;

  // If the index is 0, the address was not there in the first place.
  if (freed_idx == 0)
    return;

  // The cell at the index is now free, remove it from the linked list for
  // |Hash(address)|.
  Cell* freed_cell = &cells_[freed_idx];
  *idx_ptr = freed_cell->next;

  // Put the free cell at the front of the free list.
  freed_cell->next = free_list_;
  free_list_ = freed_idx;

  // Reset the address, so that on iteration the free cell is ignored.
  const_cast<void*&>(freed_cell->allocation.address) = nullptr;
}

AllocationRegister::Allocation* AllocationRegister::Get(void* address) {
  CellIndex* idx_ptr = Lookup(address);

  // If the index is 0, the address is not present in the table.
  return *idx_ptr == 0 ? nullptr : &cells_[*idx_ptr].allocation;
}

AllocationRegister::ConstIterator AllocationRegister::begin() const {
  // Initialize the iterator's index to 0. Cell 0 never stores an entry.
  ConstIterator iterator(*this, 0);
  // Incrementing will advance the iterator to the first used cell.
  ++iterator;
  return iterator;
}

AllocationRegister::ConstIterator AllocationRegister::end() const {
  // Cell |next_unused_cell_ - 1| is the last cell that could contain an entry,
  // so index |next_unused_cell_| is an iterator past the last element, in line
  // with the STL iterator conventions.
  return ConstIterator(*this, next_unused_cell_);
}

AllocationRegister::ConstIterator::ConstIterator(
    const AllocationRegister& alloc_register,
    CellIndex index)
    : register_(alloc_register), index_(index) {}

void AllocationRegister::ConstIterator::operator++() {
  // Find the next cell with a non-null address until all cells that could
  // possibly be used have been iterated. A null address indicates a free cell.
  do {
    index_++;
  } while (index_ < register_.next_unused_cell_ &&
           register_.cells_[index_].allocation.address == nullptr);
}

bool AllocationRegister::ConstIterator::operator!=(
    const ConstIterator& other) const {
  return index_ != other.index_;
}

const AllocationRegister::Allocation& AllocationRegister::ConstIterator::
operator*() const {
  return register_.cells_[index_].allocation;
}

AllocationRegister::CellIndex* AllocationRegister::Lookup(void* address) {
  // The list head is in |buckets_| at the hash offset.
  CellIndex* idx_ptr = &buckets_[Hash(address)];

  // Chase down the list until the cell that holds |address| is found,
  // or until the list ends.
  while (*idx_ptr != 0 && cells_[*idx_ptr].allocation.address != address)
    idx_ptr = &cells_[*idx_ptr].next;

  return idx_ptr;
}

AllocationRegister::CellIndex AllocationRegister::GetFreeCell() {
  // First try to re-use a cell from the freelist.
  if (free_list_) {
    CellIndex idx = free_list_;
    free_list_ = cells_[idx].next;
    return idx;
  }

  // Otherwise pick the next cell that has not been touched before.
  CellIndex idx = next_unused_cell_;
  next_unused_cell_++;

  // If the hash table has too little capacity (when too little address space
  // was reserved for |cells_|), |next_unused_cell_| can be an index outside of
  // the allocated storage. A guard page is allocated there to crash the
  // program in that case. There are alternative solutions:
  // - Deal with it, increase capacity by reallocating |cells_|.
  // - Refuse to insert and let the caller deal with it.
  // Because free cells are re-used before accessing fresh cells with a higher
  // index, and because reserving address space without touching it is cheap,
  // the simplest solution is to just allocate a humongous chunk of address
  // space.

  DCHECK_LT(next_unused_cell_, num_cells_ + 1);

  return idx;
}

// static
uint32_t AllocationRegister::Hash(void* address) {
  // The multiplicative hashing scheme from [Knuth 1998]. The value of |a| has
  // been chosen carefully based on measurements with real-word data (addresses
  // recorded from a Chrome trace run). It is the first prime after 2^17. For
  // |shift|, 13, 14 and 15 yield good results. These values are tuned to 2^18
  // buckets. Microbenchmarks show that this simple scheme outperforms fancy
  // hashes like Murmur3 by 20 to 40 percent.
  const uintptr_t key = reinterpret_cast<uintptr_t>(address);
  const uintptr_t a = 131101;
  const uintptr_t shift = 14;
  const uintptr_t h = (key * a) >> shift;
  return static_cast<uint32_t>(h) & kNumBucketsMask;
}

void AllocationRegister::EstimateTraceMemoryOverhead(
    TraceEventMemoryOverhead* overhead) const {
  // Estimate memory overhead by counting all of the cells that have ever been
  // touched. Don't report mmapped memory as allocated, because it has not been
  // allocated by malloc.
  size_t allocated = sizeof(AllocationRegister);
  size_t resident = sizeof(AllocationRegister)
                    // Include size of touched cells (size of |*cells_|).
                    + sizeof(Cell) * next_unused_cell_
                    // Size of |*buckets_|.
                    + sizeof(CellIndex) * kNumBuckets;
  overhead->Add("AllocationRegister", allocated, resident);
}

}  // namespace trace_event
}  // namespace base