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 bf5d0c1f399e402aba56691b593a1990ef7827f3..b7cb3c84af08579e808ab69c55fdcb458d1f4a23 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -962,6 +962,26 @@ 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. Also, # must always be in [1, 8]; otherwise it is ignored by the

[gab, 2016/01/26 21:40:26]

s/Also, /Note: /

[gab, 2016/01/26 21:40:26]

"Note that the browser process has no /prefetch:# argument; as such all other
processes must have one in order to avoid polluting its profile."

(I see now that this is mentioned on kPrefetchArgumentOther, but still think
it's worth mentioning it here?)

[fdoray, 2016/01/27 20:12:19]

Done.

[fdoray, 2016/01/27 20:12:19]

Done.

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

[gab, 2016/01/26 21:40:26]

I would really really like for all of this logic to live in one place as much as
possible...

I guess now we're only splitting off two prefetch IDs which live in Chrome.

I understand how it's hard/impossible-without-breaking-dependency-rules but I'm
worried that we use some profiles in chrome/ and it's not documented here that
those shouldn't be used by content/.

Perhaps adding a comment here like this would help:

// /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.

(and obviously change chrome from using 5/6 to 6/7)

[fdoray, 2016/01/27 20:12:19]

Done.

+// /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 prevents them from
+// interfering with the prefetch profile of the browser process.

[gab, 2016/01/26 21:40:26]

How about:

"However having a valid prefetch argument for the process types is required to
prevent them from interfering with the prefetch profile of the browser process."

(i.e. add "is required" to make this statement stronger)

[fdoray, 2016/01/27 20:12:19]

Done.

+const char kPrefetchArgumentOther[] = "/prefetch:8";
+
 // Device scale factor passed to certain processes like renderers, etc.
 const char kDeviceScaleFactor[]     = "device-scale-factor";