root/content/browser/renderer_host/backing_store_manager.cc

DEFINITIONS

1. MaxNumberOfBackingStores
2. MaxBackingStoreMemory
3. ExpireBackingStoreAt
4. ExpireLastBackingStore
5. CreateCacheSpace
6. CreateBackingStore
7. ComputeTotalArea
8. GetBackingStore
9. PrepareBackingStore
10. Lookup
11. RemoveBackingStore
12. MemorySize

// Copyright (c) 2012 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 "content/browser/renderer_host/backing_store_manager.h"

#include "base/bind.h"
#include "base/command_line.h"
#include "base/containers/mru_cache.h"
#include "base/sys_info.h"
#include "content/browser/renderer_host/backing_store.h"
#include "content/browser/renderer_host/render_widget_host_impl.h"
#include "content/public/common/content_switches.h"

namespace content {
namespace {

// There are two separate caches, |large_cache| and |small_cache|.  large_cache
// is meant for large items (tabs, popup windows), while small_cache is meant
// for small items (extension popups, HTML5 notifications). The idea is that
// we'll almost always try to evict from large_cache first since small_cache
// items will tend to be visible more of the time.
typedef base::OwningMRUCache<RenderWidgetHost*, BackingStore*>
    BackingStoreCache;
BackingStoreCache* large_cache = NULL;
BackingStoreCache* small_cache = NULL;

// Threshold is based on a single large-monitor-width toolstrip.
// (32bpp, 32 pixels high, 1920 pixels wide)
// TODO(aa): The extension system no longer supports toolstrips, but we think
// this might be helping for other examples of small HTML views in Chrome.
// Maybe this cache should be redesigned to simply prefer smaller objects to
// larger ones, rather than having a fixed threshold.
// For more background, see: crbug.com/100506.
const size_t kSmallThreshold = 4 * 32 * 1920;

// Pick a large monitor size to use as a multiplier.  This is multiplied by the
// max number of large backing stores (usually tabs) to pick a ceiling on the
// max memory to use.
// TODO(erikkay) Perhaps we should actually use monitor size?  That way we
// could make an assertion like "worse case, there are two tabs in the cache".
// However, the small_cache might mess up these calculations a bit.
// TODO(erikkay) 32bpp assumption isn't great.
const size_t kMemoryMultiplier = 4 * 1920 * 1200;  // ~9MB

// The maximum number of large BackingStoreCache objects (tabs) to use.
// Use a minimum of 2, and add one for each 256MB of physical memory you have.
// Cap at 5, the thinking being that even if you have a gigantic amount of
// RAM, there's a limit to how much caching helps beyond a certain number
// of tabs. If users *really* want unlimited stores, allow it via the
// --disable-backing-store-limit flag.
static size_t MaxNumberOfBackingStores() {
  static bool unlimited = false;
  const CommandLine& command = *CommandLine::ForCurrentProcess();
  unlimited = command.HasSwitch(switches::kDisableBackingStoreLimit);


  if (unlimited) {
    // 100 isn't truly unlimited, but given that backing stores count against
    // GDI memory, it's well past any reasonable number. Many systems will
    // begin to fail in strange ways well before they hit 100 stores.
    return 100;
  } else {
    return std::min(5, 2 + (base::SysInfo::AmountOfPhysicalMemoryMB() / 256));
  }
}

// The maximum about of memory to use for all BackingStoreCache object combined.
static size_t MaxBackingStoreMemory() {
  // Compute in terms of the number of large monitor's worth of backing-store.
  return MaxNumberOfBackingStores() * kMemoryMultiplier;
}

// Expires the given |backing_store| from |cache|.
void ExpireBackingStoreAt(BackingStoreCache* cache,
                          BackingStoreCache::iterator backing_store) {
  cache->Erase(backing_store);
}

size_t ExpireLastBackingStore(BackingStoreCache* cache) {
  if (cache->size() < 1)
    return 0;

  // Crazy C++ alert: rbegin.base() is a forward iterator pointing to end(),
  // so we need to do -- to move one back to the actual last item.
  BackingStoreCache::iterator entry = --cache->rbegin().base();
  size_t entry_size = entry->second->MemorySize();
  ExpireBackingStoreAt(cache, entry);
  return entry_size;
}

void CreateCacheSpace(size_t size) {
  // Given a request for |size|, first free from the large cache (until there's
  // only one item left) and then do the same from the small cache if we still
  // don't have enough.
  while (size > 0 && (large_cache->size() > 1 || small_cache->size() > 1)) {
    BackingStoreCache* cache =
        (large_cache->size() > 1) ? large_cache : small_cache;
    while (size > 0 && cache->size() > 1) {
      size_t entry_size = ExpireLastBackingStore(cache);
      if (size > entry_size)
        size -= entry_size;
      else
        size = 0;
    }
  }
  DCHECK(size == 0);
}

// Creates the backing store for the host based on the dimensions passed in.
// Removes the existing backing store if there is one.
BackingStore* CreateBackingStore(RenderWidgetHost* host,
                                 const gfx::Size& backing_store_size) {
  // Remove any existing backing store in case we're replacing it.
  BackingStoreManager::RemoveBackingStore(host);

  if (!large_cache) {
    large_cache = new BackingStoreCache(BackingStoreCache::NO_AUTO_EVICT);
    small_cache = new BackingStoreCache(BackingStoreCache::NO_AUTO_EVICT);
  }

  // TODO(erikkay) 32bpp is not always accurate
  size_t new_mem = backing_store_size.GetArea() * 4;
  size_t current_mem = BackingStoreManager::MemorySize();
  size_t max_mem = MaxBackingStoreMemory();
  DCHECK(new_mem < max_mem);
  if (current_mem + new_mem > max_mem) {
    // Need to remove old backing stores to make room for the new one. We
    // don't want to do this when the backing store is being replace by a new
    // one for the same WebContents, but this case won't get called then: we'll
    // have removed the old one in the RemoveBackingStore above, and the cache
    // won't be over-sized.
    CreateCacheSpace((current_mem + new_mem) - max_mem);
  }
  DCHECK((BackingStoreManager::MemorySize() + new_mem) <= max_mem);

  BackingStoreCache* cache;
  if (new_mem > kSmallThreshold) {
    // Limit the number of large backing stores (tabs) to the memory tier number
    // (between 2-5). While we allow a larger amount of memory for people who
    // have large windows, this means that those who use small browser windows
    // won't ever cache more than 5 tabs, so they pay a smaller memory cost.
    if (large_cache->size() >= MaxNumberOfBackingStores())
      ExpireLastBackingStore(large_cache);
    cache = large_cache;
  } else {
    cache = small_cache;
  }
  BackingStore* backing_store = RenderWidgetHostImpl::From(
      host)->AllocBackingStore(backing_store_size);
  if (backing_store)
    cache->Put(host, backing_store);
  return backing_store;
}

int ComputeTotalArea(const std::vector<gfx::Rect>& rects) {
  // We assume that the given rects are non-overlapping, which is a property of
  // the paint rects generated by the PaintAggregator.
#ifndef NDEBUG
  for (size_t i = 0; i < rects.size(); ++i) {
    for (size_t j = 0; j < rects.size(); ++j) {
      if (i != j)
        DCHECK(!rects[i].Intersects(rects[j]));
    }
  }
#endif
  int area = 0;
  for (size_t i = 0; i < rects.size(); ++i)
    area += rects[i].size().GetArea();
  return area;
}

}  // namespace

// BackingStoreManager ---------------------------------------------------------

// static
BackingStore* BackingStoreManager::GetBackingStore(
    RenderWidgetHost* host,
    const gfx::Size& desired_size) {
  BackingStore* backing_store = Lookup(host);
  if (backing_store) {
    // If we already have a backing store, then make sure it is the correct
    // size.
    if (backing_store->size() == desired_size)
      return backing_store;
    backing_store = NULL;
  }

  return backing_store;
}

// static
void BackingStoreManager::PrepareBackingStore(
    RenderWidgetHost* host,
    const gfx::Size& backing_store_size,
    TransportDIB::Id bitmap,
    const gfx::Rect& bitmap_rect,
    const std::vector<gfx::Rect>& copy_rects,
    float scale_factor,
    const base::Closure& completion_callback,
    bool* needs_full_paint,
    bool* scheduled_completion_callback) {
  BackingStore* backing_store = GetBackingStore(host, backing_store_size);
  if (!backing_store) {
    // We need to get Webkit to generate a new paint here, as we
    // don't have a previous snapshot.
    if (bitmap_rect.size() != backing_store_size ||
        bitmap_rect.x() != 0 || bitmap_rect.y() != 0 ||
        ComputeTotalArea(copy_rects) != backing_store_size.GetArea() ||
        !(backing_store = CreateBackingStore(host, backing_store_size))) {
      DCHECK(needs_full_paint != NULL);
      *needs_full_paint = true;
      *scheduled_completion_callback = false;
      // Makes no sense to paint the transport dib if we are going
      // to request a full paint.
      return;
    }
  }

  backing_store->PaintToBackingStore(host->GetProcess(), bitmap,
                                     bitmap_rect, copy_rects, scale_factor,
                                     completion_callback,
                                     scheduled_completion_callback);
}

// static
BackingStore* BackingStoreManager::Lookup(RenderWidgetHost* host) {
  if (large_cache) {
    BackingStoreCache::iterator it = large_cache->Get(host);
    if (it != large_cache->end())
      return it->second;

    // This moves host to the front of the MRU.
    it = small_cache->Get(host);
    if (it != small_cache->end())
      return it->second;
  }
  return NULL;
}

// static
void BackingStoreManager::RemoveBackingStore(RenderWidgetHost* host) {
  if (!large_cache)
    return;

  BackingStoreCache* cache = large_cache;
  BackingStoreCache::iterator it = cache->Peek(host);
  if (it == cache->end()) {
    cache = small_cache;
    it = cache->Peek(host);
    if (it == cache->end())
      return;
  }
  cache->Erase(it);
}

// static
size_t BackingStoreManager::MemorySize() {
  if (!large_cache)
    return 0;

  size_t mem = 0;
  BackingStoreCache::iterator it;
  for (it = large_cache->begin(); it != large_cache->end(); ++it)
    mem += it->second->MemorySize();

  for (it = small_cache->begin(); it != small_cache->end(); ++it)
    mem += it->second->MemorySize();

  return mem;
}

}  // namespace content