RFC: use some in-memory state in SimpleCache to quickly cache-miss some CantConditionalize cases

net/disk_cache/simple/simple_index.h

Index: net/disk_cache/simple/simple_index.h
diff --git a/net/disk_cache/simple/simple_index.h b/net/disk_cache/simple/simple_index.h
index 6740c2c91e2b810caa5d2092d93baff6b616c85e..a0e7740a7a8bbeaa26da165b4e9e4066019132f3 100644
--- a/net/disk_cache/simple/simple_index.h
+++ b/net/disk_cache/simple/simple_index.h
@@ -58,9 +58,12 @@ class NET_EXPORT_PRIVATE EntryMetadata {
   uint32_t GetEntrySize() const;
   void SetEntrySize(base::StrictNumeric<uint32_t> entry_size);
 
+  uint8_t GetMemoryEntryData() const { return memory_entry_data_; }
+  void SetMemoryEntryData(uint8_t val) { memory_entry_data_ = val; }
+
   // Serialize the data into the provided pickle.
   void Serialize(base::Pickle* pickle) const;
-  bool Deserialize(base::PickleIterator* it);
+  bool Deserialize(base::PickleIterator* it, bool has_memory_entry_data);
 
   static base::TimeDelta GetLowerEpsilonForTimeComparisons() {
     return base::TimeDelta::FromSeconds(1);
@@ -79,7 +82,9 @@ class NET_EXPORT_PRIVATE EntryMetadata {
   // are originally calculated as >32-bit types, the actual necessary size for
   // each shouldn't exceed 32 bits, so we use 32-bit types here.
   uint32_t last_used_time_seconds_since_epoch_;
-  uint32_t entry_size_;  // Storage size in bytes.
+  // Storage size in 256-byte blocks

[pasko, 2017/06/13 14:30:05]

4K blocks, if rounded upwards would probably be even more representative on how
much this occupies on the filesystem. WDYT?

[Maks Orlovich, 2017/06/13 14:58:47]

Hmm, maybe. Plus: it would also prevent things from changing again if we steal a
bit somewhere, and it's somewhat less arbitrary. 

OTOH, it would make cache utilization (even) less. 

@jkarlin: you know things about how cache size affects things, don't you?

+  uint32_t entry_size_ : 24;
+  uint8_t memory_entry_data_ : 8;

[pasko, 2017/06/13 14:30:05]

it would be cleaner to reserve one bit that says we did not loose the data
because of the issues with the index file. It will help with metrics and making
sure that bits set to 0 are meaningful sometimes.

[Maks Orlovich, 2017/06/13 14:58:47]

Hmm, maybe with a bool GetMemoryEntryData(uint64_t entry_hash, uint8_t* out)
const ? Could be, but I am worried about wasting bits. Really, 8 bits maybe too
much, but it makes for at least somewhat natural API.

 };
 static_assert(sizeof(EntryMetadata) == 8, "incorrect metadata size");
 
@@ -127,6 +132,9 @@ class NET_EXPORT_PRIVATE SimpleIndex
   // iff the entry exist in the index.
   bool UseIfExists(uint64_t entry_hash);
 
+  uint8_t GetMemoryEntryData(uint64_t entry_hash) const;
+  void SetMemoryEntryData(uint64_t entry_hash, uint8_t value);
+
   void WriteToDisk(IndexWriteToDiskReason reason);
 
   // Update the size (in bytes) of an entry, in the metadata stored in the