Add zram field trial

chrome/browser/chromeos/chrome_browser_main_chromeos.cc

Index: chrome/browser/chromeos/chrome_browser_main_chromeos.cc
diff --git a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
index 5b947a9e19b4326db91819aa26438e4bca14b706..99d3bddec89f6dfadacee93f8344ab4ecb104f95 100644
--- a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
+++ b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
@@ -12,6 +12,7 @@
 #include "base/callback.h"
 #include "base/chromeos/chromeos_version.h"
 #include "base/command_line.h"
+#include "base/file_util.h"
 #include "base/lazy_instance.h"
 #include "base/linux_util.h"
 #include "base/message_loop.h"
@@ -717,6 +718,7 @@ void ChromeBrowserMainPartsChromeos::PostMainMessageLoopRun() {
 
 void ChromeBrowserMainPartsChromeos::SetupPlatformFieldTrials() {
   SetupLowMemoryHeadroomFieldTrial();
+  SetupZramFieldTrial();
 }
 
 void ChromeBrowserMainPartsChromeos::SetupLowMemoryHeadroomFieldTrial() {
@@ -765,4 +767,40 @@ void ChromeBrowserMainPartsChromeos::SetupLowMemoryHeadroomFieldTrial() {
   }
 }
 
+
+void ChromeBrowserMainPartsChromeos::SetupZramFieldTrial() {
+  // The dice for this experiment have been thrown at boot.  The selected group
+  // number is stored in a file.
+  const FilePath kZramGroupPath("/home/chronos/.swap_exp_enrolled");
+  std::string zram_file_content;
+  // If the file does not exist, the experiment has not started.
+  if (!file_util::ReadFileToString(kZramGroupPath, &zram_file_content))
+    return;
+  // The file contains a single significant character, possibly followed by
+  // newline.  "x" means the user has opted out.  "0" through "8" are the valid
+  // group names.  (See src/platform/init/swap-exp.conf in chromiumos repo for
+  // group meanings.)
+  std::string zram_group = zram_file_content.substring(0, 1);
+  if (zram_group.compare("x") == 0)
+    return;
+  const base::FieldTrial::Probability kDivisor = 1;  // on/off only
+  scoped_refptr<base::FieldTrial> trial =
+      base::FieldTrialList::FactoryGetFieldTrial(
+          "ZRAM", kDivisor, "default", 2013, 12, 31, NULL);
+  // Assign probability of 1 to the group Chrome OS has picked.  Assign 0 to
+  // all other choices.
+  const char* const kGroups[] = { "0", "1", "2", "3", "4", "5", "6", "7", "8" };
+  bool matched = false;
+  for (size_t i = 0; i < arraysize(kGroups); ++i) {
+    bool match = zram_group.compare(kGroups[i]) == 0;
+    trial->AppendGroup(kGroups[i], match ? 1 : 0);
+    if (match) {
+      matched = true;
+      LOG(WARNING) << "zram field trial: group " << kGroups[i];

[stevenjb, 2013/01/08 21:36:27]

nit: We should avoid WARNING spam unless it's truly important. DVLOG(1) would be
better.
Also, can't we break here, since the compare can only succeed once? Or better
yet, couldn't we just return here and eliminate the need for |matched| entirely,
warning if we complete the loop?

[James Cook, 2013/01/08 21:38:47]

I mentioned on an earlier pass that I would like to keep the warning.  It's only
one line once per start and it will really help when looking at out-of-memory
feedback reports to tell if the user is in the trial or not.

[stevenjb, 2013/01/08 21:53:15]

Will it show up on tests? We already have quite a few "one line" WARNINGs that
show up on every test run that I would like to get rid of.

[Luigi Semenzato, 2013/01/08 21:59:01]

I would also prefer to log all the time for exactly the same reasons.

Regarding early loop termination: I might get into trouble if I do that, as the
trial object would have a variable number of groups.  The AddGroup method both
adds a group and decides if that group should be selected or not.  The API is
not sufficiently clear to diverge from existing usage. (I almost got into
trouble because the AddGroup calls are in a loop...)

[Luigi Semenzato, 2013/01/08 22:06:08]

No, it won't show up on tests, unless the file /home/chronos/.swap_exp_enrolled
exists and is readable in your test environment.

[James Cook, 2013/01/08 22:07:18]

I ran it and can confirm it doesn't log in browser_tests.

[stevenjb, 2013/01/08 22:11:49]

Ah, I see, that makes sense. It might be slightly more clear as:
int probability = (zram_group.compare(kGroups[i]) == 0) ? 1 : 0;
trial->AppendGroup(kGroups[i], probability);
But it's fine as is.

[stevenjb, 2013/01/08 22:11:49]

OK, cool, then that's fine. I wish we had a LOG(ALWAYS) instead of using WARNING
for things like this. Maybe we should revive INFO, remove any remaining spammy
uses, and repurpose it? Not in this CL of course.

+    }
+  }
+  if (!matched)
+    LOG(WARNING) << "zram field trial: invalid group \"" << zram_group << "\"";
+}
+
 }  //  namespace chromeos