tor-browser

commit f450fe1ecfd2086fdd5333a805fcaf946ba4720b
parent 5525fb5db4da63cc0aeed63bb93a373e12cfa146
Author: acreskeyMoz <acreskey@mozilla.com>
Date:   Mon, 10 Nov 2025 16:23:18 +0000

Bug 1946429 - Disable RCWN (Race Cache With Network) for devices with SSDs r=jesup,necko-reviewers,valentin

To minimize unnecessary network requests, we are by default disabing RCWN unless the client explicitly has a non-ssd drive (API only exists on Windows).

Experiments show that this is a likely net benefit to navigation performance and it definitively reduces client and infrastrucure energy expenditures.

Differential Revision: https://phabricator.services.mozilla.com/D261021

Diffstat:
M
modules/libpref/init/StaticPrefList.yaml
 | 
11
+++++++----
M
netwerk/protocol/http/nsHttpChannel.cpp
 | 
43
++++++++++++++++++++++++++++++++++++++++---

2 files changed, 47 insertions(+), 7 deletions(-)
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
@@ -13412,14 +13412,17 @@
   value: false
   mirror: always
 
-# If we should attempt to race the cache and network.
+# If we should attempt to race the cache with network.
+# See also race_with_non_ssd for auto-enable behavior on Windows non-SSD drives.
 - name: network.http.rcwn.enabled
   type: bool
-#if defined(ANDROID)
   value: false
-#else
+  mirror: always
+
+# If we should attempt to race the cache with network for non-SSD drives (Windows only).
+- name: network.http.rcwn.race_with_non_ssd
+  type: bool
   value: true
-#endif
   mirror: always
 
 - name: network.http.rcwn.cache_queue_normal_threshold
diff --git a/netwerk/protocol/http/nsHttpChannel.cpp b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -104,6 +104,9 @@
 #include "mozilla/dom/ReferrerInfo.h"
 #include "mozilla/glean/DomSecurityMetrics.h"
 #include "mozilla/Telemetry.h"
+#include "mozilla/Services.h"
+#include "nsISystemInfo.h"
+#include "mozilla/Components.h"
 #include "AlternateServices.h"
 #include "NetworkMarker.h"
 #include "nsIDNSRecord.h"
@@ -4637,6 +4640,41 @@ nsresult nsHttpChannel::OpenCacheEntry(bool isHttps) {
   return OpenCacheEntryInternal(isHttps);
 }
 
+#ifdef XP_WIN
+static mozilla::Maybe<bool> sHasSSD;
+#endif
+
+static bool RCWNEnabled() {
+  // State table for RCWN logic (race_with_non_ssd is Windows only):
+  // network.http.rcwn.enabled | Device  | race_with_non_ssd | Result
+  // true                      | any     | any               | Enabled
+  // false                     | SSD     | any               | Disabled
+  // false                     | non-SSD | true              | Enabled
+  // false                     | non-SSD | false             | Disabled
+
+  bool rcwnEnabled = StaticPrefs::network_http_rcwn_enabled();
+#ifdef XP_WIN
+  if (!rcwnEnabled) {
+    if (sHasSSD.isNothing()) {
+      bool hasSSD = true;
+      nsCOMPtr<nsIPropertyBag2> sysInfo =
+          mozilla::components::SystemInfo::Service();
+      if (NS_SUCCEEDED(sysInfo->GetPropertyAsBool(u"hasSSD"_ns, &hasSSD))) {
+        sHasSSD = Some(hasSSD);
+      } else {
+        // Failed to detect, assume SSD (conservative default)
+        sHasSSD = Some(true);
+      }
+    }
+    if (sHasSSD.isSome() && !sHasSSD.value()) {
+      // For non-SSD devices, check the non-SSD-specific preference
+      rcwnEnabled = StaticPrefs::network_http_rcwn_race_with_non_ssd();
+    }
+  }
+#endif
+  return rcwnEnabled;
+}
+
 nsresult nsHttpChannel::OpenCacheEntryInternal(bool isHttps) {
   nsresult rv;
 
@@ -4771,8 +4809,7 @@ nsresult nsHttpChannel::OpenCacheEntryInternal(bool isHttps) {
     maybeRCWN = false;
   }
 
-  if ((mNetworkTriggerDelay || StaticPrefs::network_http_rcwn_enabled()) &&
-      maybeRCWN && mAllowRCWN) {
+  if ((mNetworkTriggerDelay || RCWNEnabled()) && maybeRCWN && mAllowRCWN) {
     bool hasAltData = false;
     uint32_t sizeInKb = 0;
     rv = cacheStorage->GetCacheIndexEntryAttrs(
@@ -4790,7 +4827,7 @@ nsresult nsHttpChannel::OpenCacheEntryInternal(bool isHttps) {
   if (!mCacheOpenDelay) {
     MOZ_ASSERT(NS_IsMainThread(), "Should be called on the main thread");
     if (mNetworkTriggered) {
-      mRaceCacheWithNetwork = StaticPrefs::network_http_rcwn_enabled();
+      mRaceCacheWithNetwork = RCWNEnabled();
     }
     rv = cacheStorage->AsyncOpenURI(mCacheEntryURI, mCacheIdExtension,
                                     cacheEntryOpenFlags, this);