evict larger entries first

net/disk_cache/simple/simple_index.cc

Index: net/disk_cache/simple/simple_index.cc
diff --git a/net/disk_cache/simple/simple_index.cc b/net/disk_cache/simple/simple_index.cc
index c41c829bdefd808db4ea55dded03e54f4574b641..f207e576da9fecb60dc755e15c3ec2187095fbc5 100644
--- a/net/disk_cache/simple/simple_index.cc
+++ b/net/disk_cache/simple/simple_index.cc
@@ -11,6 +11,7 @@
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
+#include "base/bits.h"
 #include "base/files/file_enumerator.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
@@ -25,6 +26,7 @@
 #include "base/trace_event/memory_usage_estimator.h"
 #include "net/base/net_errors.h"
 #include "net/disk_cache/simple/simple_entry_format.h"
+#include "net/disk_cache/simple/simple_experiment.h"
 #include "net/disk_cache/simple/simple_histogram_macros.h"
 #include "net/disk_cache/simple/simple_index_delegate.h"
 #include "net/disk_cache/simple/simple_index_file.h"
@@ -49,6 +51,13 @@ const uint32_t kEvictionMarginDivisor = 20;
 
 const uint32_t kBytesInKb = 1024;
 
+// This is added to the size of each entry before using the size
+// to determine which entries to evict first. It's basically an
+// estimate of the filesystem overhead, but it also serves to flatten
+// the curve so that 1-byte entries and 2-byte entries are basically
+// treated the same.
+static const int kEstimatedEntryOverhead = 512;
+
 }  // namespace
 
 namespace disk_cache {
@@ -309,31 +318,81 @@ void SimpleIndex::StartEvictionIfNeeded() {
       static_cast<base::HistogramBase::Sample>(max_size_ / kBytesInKb));
 
   // Flatten for sorting.
-  std::vector<const std::pair<const uint64_t, EntryMetadata>*> entries;
+  typedef std::pair<const uint64_t, EntryMetadata> Entry;
+  typedef std::pair<uint64_t, const Entry*> SortHelper;
+  std::vector<SortHelper> entries;
   entries.reserve(entries_set_.size());
+  uint32_t now = (base::Time::Now() - base::Time::UnixEpoch()).InSeconds();
+  bool use_size = base::FeatureList::IsEnabled(kSimpleCacheEvictionWithSize);
   for (EntrySet::const_iterator i = entries_set_.begin();
        i != entries_set_.end(); ++i) {
-    entries.push_back(&*i);
+    uint64_t sort_value = now - i->second.RawTimeForSorting();
+    if (use_size)
+      sort_value *= i->second.GetEntrySize() + kEstimatedEntryOverhead;
+    // Subtract so we don't need a custom comparator.
+    entries.emplace_back(std::numeric_limits<uint64_t>::max() - sort_value,
+                         &*i);
   }
 
-  std::sort(entries.begin(), entries.end(),
-            [](const std::pair<const uint64_t, EntryMetadata>* a,
-               const std::pair<const uint64_t, EntryMetadata>* b) -> bool {
-              return a->second.RawTimeForSorting() <
-                     b->second.RawTimeForSorting();
-            });
-
-  // Remove as many entries from the index to get below |low_watermark_|,
-  // collecting least recently used hashes into |entry_hashes|.
-  std::vector<uint64_t> entry_hashes;
-  std::vector<const std::pair<const uint64_t, EntryMetadata>*>::iterator it =
-      entries.begin();
+  // introselect algorithm:
+  // Pick a pivot element, partition elements into elements
+  // smaller than the pivot and not smaller than pivot.
+  // Then sum all the sizes from the "smaller" bucket. If it
+  // is still too large, then iterate again, but only consider
+  // elements from the "smaller" bucket.  If not, add all elements
+  // in the "smaller" bucket to the evict list.
+  // Then iterate again, to partition the "not smaller" bucket.
+  // If we go over 2 * log2(|entries|) iterations, resort to calling
+  // std::sort() to avoid N-squared worst case scenario.
   uint64_t evicted_so_far_size = 0;
-  while (evicted_so_far_size < cache_size_ - low_watermark_) {
-    DCHECK(it != entries.end());
-    entry_hashes.push_back((*it)->first);
-    evicted_so_far_size += (*it)->second.GetEntrySize();
-    ++it;
+  const uint64_t amount_to_evict = cache_size_ - low_watermark_;
+  std::vector<uint64_t> entry_hashes;
+  int loops = 2 * base::bits::Log2Ceiling(entries.size());
+  auto begin = entries.begin();
+  auto end = entries.end();
+  while (loops > 0 && evicted_so_far_size < amount_to_evict &&
+         end - begin >= 3) {
+    loops--;
+
+    // Select a pivot.
+    uint64_t tmp[3];
+    tmp[0] = begin->first;
+    tmp[1] = (end - 1)->first;
+    tmp[2] = (begin + (end - begin) / 2)->first;
+    std::sort(tmp, tmp + 3);
+    uint64_t pivot = tmp[1];
+
+    // Partition
+    auto middle = std::partition(begin, end, [pivot](const SortHelper& entry) {
+      return entry.first < pivot;
+    });
+    if (middle == end || middle == begin) {
+      // No progress will be made.
+      break;
+    }
+    uint64_t lt_sum = 0;
+    for (auto i = begin; i != middle; ++i)
+      lt_sum += i->second->second.GetEntrySize();
+
+    if (evicted_so_far_size + lt_sum > amount_to_evict) {
+      end = middle;
+    } else {
+      evicted_so_far_size += lt_sum;
+      for (auto i = begin; i != middle; ++i)
+        entry_hashes.push_back(i->second->first);
+
+      begin = middle;
+    }
+  }
+
+  if (end > begin && evicted_so_far_size < amount_to_evict) {
+    std::sort(begin, end);
+    for (auto i = begin; i < end; ++i) {
+      if (evicted_so_far_size >= amount_to_evict)
+        break;
+      evicted_so_far_size += i->second->second.GetEntrySize();
+      entry_hashes.push_back(i->second->first);
+    }
   }
 
   SIMPLE_CACHE_UMA(COUNTS,