[page_load_metrics] Log cache warmth ratios at parse stop

chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
index 4cdc73f92eb24c8bb6258533f2fccfd523006ca1..fd9c7f73a5890290f091338a2b43bfc53d72ea2d 100644
--- a/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
@@ -158,6 +158,11 @@ const char kHistogramFailedProvisionalLoad[] =
 const char kHistogramForegroundToFirstPaint[] =
     "PageLoad.PaintTiming.ForegroundToFirstPaint";
 
+const char kHistogramCacheRequestPercentParseStop[] =
+    "PageLoad.Cache.RequestPercent.ParseStop";

[Bryan McQuade, 2016/08/08 18:53:28]

can we call these 'PageLoad.Experimental.Cache' for now? once we're happy with
them we can promote them out of experimental.

[Charlie Harrison, 2016/08/08 20:55:03]

Done.

+const char kHistogramCacheBytePercentParseStop[] =
+    "PageLoad.Cache.BytePercent.ParseStop";
+
 const char kRapporMetricsNameCoarseTiming[] =
     "PageLoad.CoarseTiming.NavigationToFirstContentfulPaint";
 
@@ -356,6 +361,18 @@ void CorePageLoadMetricsObserver::OnParseStop(
         internal::kHistogramParseBlockedOnScriptLoadDocumentWrite,
         timing.parse_blocked_on_script_load_from_document_write_duration
             .value());
+
+    int total_requests = info.cache_requests + info.network_requests;
+    int64_t total_bytes =
+        info.cache_subresource_bytes + info.network_subresource_bytes;
+    if (total_requests && total_bytes) {
+      UMA_HISTOGRAM_PERCENTAGE(internal::kHistogramCacheRequestPercentParseStop,

[Bryan McQuade, 2016/08/08 18:53:28]

one tricky thing with percentages is if you get something like 1/2 that's 50%
which ends up being the same as 25/50 - I'd claim that a page with 25 resources
served from cache has very different performance characteristics from a page
that has 1 served out of cache. To get better insight here, can we also add some
counters? It'd be interesting for instance to count number of pages that have a
very small number of total resources - we might want to discard those pages from
the percentage histogram.

+                               (100 * info.cache_requests) / total_requests);
+      UMA_HISTOGRAM_PERCENTAGE(
+          internal::kHistogramCacheBytePercentParseStop,
+          (100 * info.cache_subresource_bytes) / total_bytes);
+    }
+
   } else {
     PAGE_LOAD_HISTOGRAM(internal::kBackgroundHistogramParseDuration,
                         parse_duration);