Use valid /prefetch arguments for process launches on Windows. - do not submit

chrome/common/prefetch_argument_win.h

Index: chrome/common/prefetch_argument_win.h
diff --git a/chrome/common/prefetch_argument_win.h b/chrome/common/prefetch_argument_win.h
new file mode 100644
index 0000000000000000000000000000000000000000..7422d996784cd8b509d1f7baf959bb5ebfd8ab30
--- /dev/null
+++ b/chrome/common/prefetch_argument_win.h
@@ -0,0 +1,25 @@
+// Copyright (c) 2016 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.
+
+#ifndef CHROME_COMMON_PREFETCH_ARGUMENT_WIN_H_
+#define CHROME_COMMON_PREFETCH_ARGUMENT_WIN_H_
+
+namespace base {
+class CommandLine;
+}  // namespace base
+
+namespace chrome {
+// Adds a prefetch argument to |command_line|. The --type and
+// --no-startup-window switches must already have been added, if applicable, as

[gab, 2016/01/18 19:10:52]

I find it weird to have to document which flags have to have been added already.

And since I'm suggesting somewhere else that we move this to components you
won't have access to those switches constants anyways...

So how about we instead have an enum here of every process type we handle and
the caller tells us which one it is?

+// they determine which prefetch argument is added to |command_line|. It has
+// been observed that when file reads are consistent for 3 process launches with
+// the same prefetch argument, the prefetcher starts issuing the reads in batch
+// at process launch. This is more efficient than reading a few pages at the
+// time on-demand. The prefetcher wouldn't be able to observe consistent file

[gab, 2016/01/18 19:10:52]

s/at the time/at a time/

[gab, 2016/01/18 19:10:52]

s/wouldn't/isn't/

+// reads across Chrome process launches if no prefetch argument was used,
+// because the reads depend on the process type.
+void AddWindowsPrefetchArgument(base::CommandLine* command_line);

[gab, 2016/01/18 19:10:52]

To ensure this is called by everyone creating a chrome.exe process how about we
add a DCHECK in chrome.exe's main that it either identifies itself as a browser
process or there is a /prefetch flag on its command-line?

That should catch any new process that forgets to calls this when forging its
command-line?

+}  // namespace chrome
+
+#endif  // CHROME_COMMON_PREFETCH_ARGUMENT_WIN_H_