Use a valid /prefetch argument when launching a process.

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index b87df7f8f2d4a75c01367d7858920e4ec9fd1d76..c7c03a4b2f085a2a73d18d281ba9e309761ca1da 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -966,6 +966,32 @@ const char kDisableCoreAnimationPlugins[] =
 #endif
 
 #if defined(OS_WIN)
+// /prefetch:# arguments to use when launching various process types. It has
+// been observed that when file reads are consistent for 3 process launches with
+// the same /prefetch:# argument, the Windows prefetcher starts issuing reads in
+// batch at process launch. Because reads depend on the process type, the
+// prefetcher wouldn't be able to observe consistent reads if no /prefetch:#
+// arguments were used. Note that the browser process has no /prefetch:#
+// argument; as such all other processes must have one in order to avoid
+// polluting its profile. Note: # must always be in [1, 8]; otherwise it is
+// ignored by the// Windows prefetcher.

[gab, 2016/01/29 19:39:28]

rm "//" mid sentence.

[fdoray, 2016/02/01 13:28:26]

Done.

+const char kPrefetchArgumentRenderer[] = "/prefetch:1";
+const char kPrefetchArgumentGpu[] = "/prefetch:2";
+const char kPrefetchArgumentPpapi[] = "/prefetch:3";
+const char kPrefetchArgumentPpapiBroker[] = "/prefetch:4";
+

[gab, 2016/01/29 19:39:28]

I'd remove this empty line to make it even clearer that the follow-up comments
are about the next IDs.

[fdoray, 2016/02/01 13:28:26]

Done.

+// /prefetch:5 is unused to this date and can be claimed by a new process type.
+// /prefetch:6 and /prefetch:7 are reserved for content embedders and are not to
+// be used by content itself.
+
+// /prefetch:# argument shared by all process types that don't have their own.
+// It is likely that the prefetcher won't work for these process types as it
+// won't be able to observe consistent file reads across launches. However,
+// having a valid prefetch argument for these process types is required to
+// prevent them from interfering with the prefetch profile of the browser
+// process.
+const char kPrefetchArgumentOther[] = "/prefetch:8";
+
 // Device scale factor passed to certain processes like renderers, etc.
 const char kDeviceScaleFactor[]     = "device-scale-factor";