Remove all code related to the PreRead field trial.

chrome/app/file_pre_reader_win.cc

Index: chrome/app/file_pre_reader_win.cc
diff --git a/chrome/app/file_pre_reader_win.cc b/chrome/app/file_pre_reader_win.cc
index f95c0276b5dbff0ff4b1e5ace5e6015842ef31b4..29c90e1256866ec85ad313e139d4a1557e56f247 100644
--- a/chrome/app/file_pre_reader_win.cc
+++ b/chrome/app/file_pre_reader_win.cc
@@ -7,16 +7,8 @@
 #include <windows.h>
 
 #include "base/files/file.h"
-#include "base/files/memory_mapped_file.h"
-#include "base/logging.h"
-#include "base/threading/platform_thread.h"
-#include "base/threading/thread_restrictions.h"
-#include "components/startup_metric_utils/common/pre_read_field_trial_utils_win.h"
 
-namespace {
-
-// Pre-reads |file_path| using ::ReadFile.
-void PreReadFileUsingReadFile(const base::FilePath& file_path) {
+void PreReadFile(const base::FilePath& file_path) {
   base::File file(file_path, base::File::FLAG_OPEN | base::File::FLAG_READ |
                                  base::File::FLAG_SEQUENTIAL_SCAN);
   if (!file.IsValid())
@@ -32,62 +24,3 @@ void PreReadFileUsingReadFile(const base::FilePath& file_path) {
 
   ::VirtualFree(buffer, 0, MEM_RELEASE);
 }
-
-// Pre-reads |file_path| using ::PrefetchVirtualMemory, if available. Otherwise,
-// falls back on using ::ReadFile.
-void PreReadFileUsingPrefetchVirtualMemory(const base::FilePath& file_path) {
-  // Load ::PrefetchVirtualMemory dynamically, because it is only available on
-  // Win8+.
-  using PrefetchVirtualMemoryPtr = decltype(::PrefetchVirtualMemory)*;
-  PrefetchVirtualMemoryPtr prefetch_virtual_memory =
-      reinterpret_cast<PrefetchVirtualMemoryPtr>(::GetProcAddress(
-          ::GetModuleHandle(L"kernel32.dll"), "PrefetchVirtualMemory"));

[gab, 2016/08/31 16:39:57]

Interesting, so the built-in Windows prefetcher wasn't as good as a basic
buffered file read?

[fdoray, 2016/08/31 19:50:52]

From ETW traces, it looks like PrefetchVirtualMemory does the same thing as us.
At the 75th percentile, I got a 1% +/- 6% improvement when using
PrefetchVirtualMemory (week ending on February 8th) vs buffered file reads. I'm
not convinced that this justifies the added complexity of having 2 code paths
(Win7 vs Win8+).

[gab, 2016/08/31 19:54:47]

Ok, then I'd say add a comment mentioning that this would be a good replacement
when we drop support for Win7 with an explanation of the results (so someone
doesn't come in and redo the same experiment later).

[fdoray, 2016/08/31 20:00:33]

Done.

-  if (!prefetch_virtual_memory) {
-    // If ::PrefetchVirtualMemory is not available, fall back to
-    // PreReadFileUsingReadFile().
-    PreReadFileUsingReadFile(file_path);
-    return;
-  }
-
-  base::MemoryMappedFile memory_mapped_file;
-  if (!memory_mapped_file.Initialize(file_path)) {
-    // Initializing the memory map should not fail. If it does fail in a debug
-    // build, we want to be warned about it so that we can investigate the
-    // failure.
-    NOTREACHED();
-    PreReadFileUsingReadFile(file_path);
-    return;
-  }
-
-  WIN32_MEMORY_RANGE_ENTRY memory_range;
-  memory_range.VirtualAddress = const_cast<void*>(
-      reinterpret_cast<const void*>(memory_mapped_file.data()));
-  memory_range.NumberOfBytes = memory_mapped_file.length();
-  prefetch_virtual_memory(::GetCurrentProcess(), 1U, &memory_range, 0);
-}
-
-}  // namespace
-
-void PreReadFile(const base::FilePath& file_path,
-                 const startup_metric_utils::PreReadOptions& pre_read_options) {
-  DCHECK(pre_read_options.pre_read);
-  base::ThreadRestrictions::AssertIOAllowed();
-
-  // Increase thread priority if necessary.
-  base::ThreadPriority previous_priority = base::ThreadPriority::NORMAL;
-  if (pre_read_options.high_priority) {
-    previous_priority = base::PlatformThread::GetCurrentThreadPriority();
-    base::PlatformThread::SetCurrentThreadPriority(
-        base::ThreadPriority::DISPLAY);
-  }
-
-  // Pre-read |file_path|.
-  if (pre_read_options.prefetch_virtual_memory)
-    PreReadFileUsingPrefetchVirtualMemory(file_path);
-  else
-    PreReadFileUsingReadFile(file_path);
-
-  // Reset thread priority.
-  if (pre_read_options.high_priority)
-    base::PlatformThread::SetCurrentThreadPriority(previous_priority);
-}