Minor SimpleCacheBackend improvements.

net/disk_cache/simple/simple_backend_impl.h

Index: net/disk_cache/simple/simple_backend_impl.h
diff --git a/net/disk_cache/simple/simple_backend_impl.h b/net/disk_cache/simple/simple_backend_impl.h
index ebb96c7caca6a14d750108aed74eea9164ad109b..9f1e90d3806eabc07067e7a86076104c7fe05e5a 100644
--- a/net/disk_cache/simple/simple_backend_impl.h
+++ b/net/disk_cache/simple/simple_backend_impl.h
@@ -12,6 +12,7 @@
 
 #include "base/compiler_specific.h"
 #include "base/files/file_path.h"
+#include "base/hash_tables.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
@@ -84,7 +85,7 @@ class NET_EXPORT_PRIVATE SimpleBackendImpl : public Backend,
   virtual void OnExternalCacheHit(const std::string& key) OVERRIDE;
 
  private:
-  typedef std::map<uint64, base::WeakPtr<SimpleEntryImpl> > EntryMap;
+  typedef base::hash_map<uint64, base::WeakPtr<SimpleEntryImpl> > EntryMap;

[pasko, 2013/05/17 10:31:33]

This change cannot make any significant improvement: the amount of active
entries is not more than 300 at all times. Even worse, it can degrade
performance if a 'malicious' website attacks our hash function making buckets
for google.com main resource very lengthy, for example.

I would rather leave it as is.

[digit1, 2013/05/17 14:36:18]

I feel that these statements are contradicting each other :)

If there are no more than 300 entries at all times, then a degenerate map will
result in a linked list with 300 items in it. Even a mobile CPU should be able
to scan such a list and perform 64-bit comparisons in each node in a _very_
reasonable amount of time, so the effect of such denial of service would
probably not be noticeable.

Moreover, I though we were using a cryptographic hash like SHA-1 to make this
DOS attack extremely difficult to pull. I.e. I don't think there is an easy way
to produce hundreds or even thousands of input strings that all hash to the same
SHA-1. Even if there was, this would require several hundreds / thousands
concurrent active requests, right?

If you still think a std::map() is better, I propose adding a comment on top of
this typedef explaining why this container type is preferred.

[pasko, 2013/05/17 16:57:19]

You are right, there is practically no difference. Keep it whatever you like,
just remove "include <map>" if you decide to stay with hash_map.

 
   typedef base::Callback<void(uint64 max_size, int result)>
       InitializeIndexCallback;