Bug 1573904 - Measure the potential for a streaming parser. data-review=chutten r=jorendorff,chutten

Differential Revision: https://phabricator.services.mozilla.com/D42039

--HG--
extra : moz-landing-system : lando

dom/script/ScriptLoadHandler.cpp

@@ -71,6 +71,22 @@ nsresult ScriptLoadHandler::DecodeRawDataHelper(const uint8_t* aData,
       MakeSpan(scriptText.begin() + haveRead, needed.value()), aEndOfStream);
   MOZ_ASSERT(written <= needed.value());
 
+  // Telemetry: Measure the throughput at which bytes are streamed and the ratio
+  // of streamed bytes, such that we can determine the effectiveness of a
+  // streaming parser for JavaScript.
+  using namespace mozilla::Telemetry;
+  if (aEndOfStream && haveRead) {
+    // Compute the percent of data transfered incrementally.
+    Accumulate(DOM_SCRIPT_LOAD_INCREMENTAL_RATIO, 100 * haveRead / (haveRead + written));
+    // Compute the rate of transfer of the incremental data calls averaged
+    // across the time needed to complete the request.
+    double ms = (TimeStamp::Now() - mFirstOnIncrementalData).ToMilliseconds();
+    Accumulate(DOM_SCRIPT_LOAD_INCREMENTAL_AVG_TRANSFER_RATE, haveRead / ms);
+  }
+  if (!aEndOfStream && !haveRead) {
+    mFirstOnIncrementalData = TimeStamp::Now();
+  }
+
   haveRead += written;
   MOZ_ASSERT(haveRead <= capacity.value(),
              "mDecoder produced more data than expected");

dom/script/ScriptLoadHandler.h

@@ -12,6 +12,7 @@
 #define mozilla_dom_ScriptLoadHandler_h
 
 #include "nsIIncrementalStreamLoader.h"
+#include "mozilla/TimeStamp.h"
 
 namespace mozilla {
 namespace dom {
@@ -97,6 +98,9 @@ class ScriptLoadHandler final : public nsIIncrementalStreamLoaderObserver {
 
   // Unicode decoder for charset.
   mozilla::UniquePtr<mozilla::Decoder> mDecoder;
+
+  // Used to compute the DOM_SCRIPT_LOAD_INCREMENTAL_AVG_TRANSFER_RATE telemetry.
+  TimeStamp mFirstOnIncrementalData;
 };
 
 }  // namespace dom

toolkit/components/telemetry/Histograms.json

@@ -13702,6 +13702,30 @@
     "labels": ["Used", "RejectedByPolicy", "RequestMismatch", "LoadError", "NotUsed"],
     "description": "Whether a preloaded script was used or the reason it was not used."
   },
+  "DOM_SCRIPT_LOAD_INCREMENTAL_RATIO": {
+    "record_in_processes": ["main", "content"],
+    "products": ["firefox", "fennec", "geckoview"],
+    "alert_emails": ["nicolas.b.pierron@mozilla.com"],
+    "expires_in_version": "75",
+    "kind": "linear",
+    "low": 2,
+    "high": 100,
+    "n_buckets": 99,
+    "bug_numbers": [1573904],
+    "description": "Ratio of JavaScript-text bytes transferred incrementally compared to the total number of bytes transferred."
+  },
+  "DOM_SCRIPT_LOAD_INCREMENTAL_AVG_TRANSFER_RATE": {
+    "record_in_processes": ["main", "content"],
+    "products": ["firefox", "fennec", "geckoview"],
+    "alert_emails": ["nicolas.b.pierron@mozilla.com"],
+    "expires_in_version": "75",
+    "kind": "exponential",
+    "low": 1,
+    "high": 1000000,
+    "n_buckets": 60,
+    "bug_numbers": [1573904],
+    "description": "Speed at which JavaScript-text bytes are transferred incrementally, measured between the first onIncremantalData call and the onStreamComplete call (in character/ms)."
+  },
   "VIDEO_DROPPED_FRAMES_PROPORTION" : {
     "record_in_processes": ["content"],
     "products": ["firefox", "geckoview"],