SimpleCache: read small files all at once.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 112498ffcd8dfb2f57d62214a51a4f1456111b27..76fb53f71259b08710743a894fc1ae1518639a3e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -73448,6 +73448,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>The outcome of Entry::ReadData in the simple cache.</summary>
 </histogram>
 
+<histogram base="true" name="SimpleCache.ReadStream1FromPrefetched"
+    enum="Boolean">
+  <owner>morlovich@chromium.org</owner>
+  <summary>
+    Whether a read from stream 1 was satisfied from prefetch data. Reported only
+    on first read op.

[pasko, 2017/08/04 01:28:35]

s!first read op.!first read op of each entry!

Also: if there are multiple concurrent openers/readers/writers of the entry,
only the result from a single read is recorded. Maybe it'd be nice to mention
that?

[Maks Orlovich, 2017/08/04 18:35:44]

Yeah, good point. I think they're pretty rare, though, since that sounds kinda
hard to interpret.

[pasko, 2017/08/09 12:28:21]

well, if they were too rare, we would not have invested into dealing with the
cache lock. These edge cases are hard to interpret and remember, that's why I
think being more explicit in a description here would help a lot.

[Maks Orlovich, 2017/08/09 15:25:01]

Oh, wow, I thought I changed this... Huh, when I open the file in my source tree
I get this:

"
    Whether a read from stream 1 was satisfied from prefetch data. Reported only
    on first read op of entry (including if there are multiple readers, or even
    some writers).
"
Not sure what's going on here.

[pasko, 2017/08/09 15:33:58]

Thanks! When I was replying I did not look at your latest changes because your
response did not obviously suggest that you made the changes.

Confusionz :)

+  </summary>
+</histogram>
+
 <histogram base="true" name="SimpleCache.StaleIndexExtraEntryCount"
     units="entries">
   <owner>gavinp@chromium.org</owner>
@@ -73530,6 +73539,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram base="true" name="SimpleCache.SyncOpenDidPrefetch" enum="Boolean">
+  <owner>morlovich@chromium.org</owner>
+  <summary>
+    Whether an attempt was made to prefetch the entire file when opening the
+    entry.
+  </summary>
+</histogram>
+
 <histogram base="true" name="SimpleCache.SyncOpenEntryAge" units="hours">
   <owner>gavinp@chromium.org</owner>
   <summary>
@@ -90395,6 +90412,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <affected-histogram name="SimpleCache.QueueLatency.OpenEntry"/>
   <affected-histogram name="SimpleCache.ReadIsParallelizable"/>
   <affected-histogram name="SimpleCache.ReadResult"/>
+  <affected-histogram name="SimpleCache.ReadStream1FromPrefetched"/>
   <affected-histogram name="SimpleCache.StaleIndexExtraEntryCount"/>
   <affected-histogram name="SimpleCache.StaleIndexMissedEntryCount"/>
   <affected-histogram name="SimpleCache.StaleIndexQuality"/>
@@ -90409,6 +90427,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <affected-histogram name="SimpleCache.SyncCreateResult_WithIndex"/>
   <affected-histogram name="SimpleCache.SyncCreateResult_WithoutIndex"/>
   <affected-histogram name="SimpleCache.SyncKeySHA256Result"/>
+  <affected-histogram name="SimpleCache.SyncOpenDidPrefetch"/>
   <affected-histogram name="SimpleCache.SyncOpenEntryAge"/>
   <affected-histogram name="SimpleCache.SyncOpenPlatformFileError"/>
   <affected-histogram name="SimpleCache.SyncOpenPlatformFileError_WithIndex"/>