Add a PartialPreReadImage function to file_util (on Windows).

base/file_util.h

 // 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.
 
 // This file contains utility functions for dealing with the local
 // filesystem.
 
 #ifndef BASE_FILE_UTIL_H_
 #define BASE_FILE_UTIL_H_
 #pragma once
@@ skipping 591 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(MemoryMappedFile);
 };
 
 // Returns whether the file has been modified since a particular date.
 BASE_EXPORT bool HasFileBeenModifiedSince(
     const FileEnumerator::FindInfo& find_info,
     const base::Time& cutoff_time);
 
 #if defined(OS_WIN)
-  // Loads the file passed in as an image section and touches pages to avoid
-  // subsequent hard page faults during LoadLibrary. The size to be pre read
-  // is passed in. If it is 0 then the whole file is paged in. The step size
-  // which indicates the number of bytes to skip after every page touched is
-  // also passed in.
+// Loads the file passed in as an image section and touches pages to avoid
+// subsequent hard page faults during LoadLibrary. The size to be pre read
+// is passed in. If it is 0 then the whole file is paged in. The step size
+// which indicates the number of bytes to skip after every page touched is
+// also passed in.
 bool BASE_EXPORT PreReadImage(const wchar_t* file_path, size_t size_to_read,
                               size_t step_size);
+
+// Loads the file passed in as an image section and touches pages to avoid
+// subsequent hard page faults during LoadLibrary. The percentage of the
+// file to be read is passed in, as an integral value between 0 and 100,
+// inclusive. If it is 0 then this is a NOP, if it is 100 (or greater) then
+// the whole file is paged in via PreReadImage. The max_chunk_size, which

[Evan Martin, 2012/01/27 20:13:58]

This isn't clear to me -- does a percentage of 50 mean it only reads the first
half of the file?  Why is percentage a size_t when it is not a size?

[Roger McFarlane (Google), 2012/01/27 21:25:54]

Clarified the comments, and switched percentage to a uint8.

+// indicates the number of bytes to read off the disk in each step (for
+// operating systems where this is the way the pages are warmed), is also
+// passed in (otherwise, the in-memory image is touched once per system page).
+bool BASE_EXPORT PartialPreReadImage(const wchar_t* file_path,

[Evan Martin, 2012/01/27 20:13:58]

Everywhere else I think we put BASE_EXPORT first.  Can you also fix these the
one function above, that you didn't change?

[Roger McFarlane (Google), 2012/01/27 21:25:54]

Done.

+                                     size_t percentage,
+                                     size_t max_chunk_size);
+namespace internal {
+
+// Helper function used by PartialPreReadImage on Windows versions (Vista+)
+// where reading through the file on disk serves to warm up the page cache.
+// Exported for unit-testing purposes.
+bool BASE_EXPORT PartialPreReadImageOnDisk(const wchar_t* file_path,
+                                           size_t percentage,
+                                           size_t max_chunk_size);
+
+// Helper function used by PartialPreReadImage on Windows versions (XP) where
+// cheaply loading the image then stepping through its address space serves
+// to warm up the page cache. Exported for unit-testing purposes.
+bool BASE_EXPORT PartialPreReadImageInMemory(const wchar_t* file_path,
+                                             size_t percentage);
+
+}  // namespace internal
+
 #endif  // OS_WIN
 
 #if defined(OS_LINUX)
 // Broad categories of file systems as returned by statfs() on Linux.
 enum FileSystemType {
   FILE_SYSTEM_UNKNOWN,  // statfs failed.
   FILE_SYSTEM_0,        // statfs.f_type == 0 means unknown, may indicate AFS.
   FILE_SYSTEM_ORDINARY,       // on-disk filesystem like ext2
   FILE_SYSTEM_NFS,
   FILE_SYSTEM_SMB,
   FILE_SYSTEM_CODA,
   FILE_SYSTEM_MEMORY,         // in-memory file system
   FILE_SYSTEM_CGROUP,         // cgroup control.
   FILE_SYSTEM_OTHER,          // any other value.
   FILE_SYSTEM_TYPE_COUNT
 };
 
 // Attempts determine the FileSystemType for |path|.
 // Returns false if |path| doesn't exist.
 BASE_EXPORT bool GetFileSystemType(const FilePath& path, FileSystemType* type);
 #endif
 
 }  // namespace file_util
 
 // Deprecated functions have been moved to this separate header file,
 // which must be included last after all the above definitions.
 #include "base/file_util_deprecated.h"
 
 #endif  // BASE_FILE_UTIL_H_