Low end devices should not prewarm Web process on provisional load

https://bugs.webkit.org/show_bug.cgi?id=259927
rdar://113557767

Reviewed by Chris Dumez.

After 266255@main, a Web process is prewarmed on provisional load instead of when the main frame load has finished.
This is a page load time progression on most iOS devices, except for the low end ones. For these devices, we should
still be prewarming when the main frame load has finished. The change in 266255@main is also performance neutral on
macOS, so we can also keep the original behavior there.

* Source/WTF/wtf/PlatformEnableCocoa.h:
* Source/WebKit/UIProcess/WebPageProxy.cpp:
(WebKit::shouldPrewarmWebProcessOnProvisionalLoad):
(WebKit::WebPageProxy::didStartProvisionalLoadForFrameShared):
(WebKit::WebPageProxy::didFinishLoadForFrame):

Canonical link: https://commits.webkit.org/266717@main

Source/WTF/wtf/PlatformEnableCocoa.h

@@ -899,3 +899,7 @@
     || PLATFORM(VISION))
 #define ENABLE_SAMPLE_BUFFER_CONTENT_KEY_SESSION_SUPPORT 1
 #endif
+
+#if !defined(ENABLE_PREWARM_WEBPROCESS_ON_PROVISIONAL_LOAD) && PLATFORM(IOS)
+#define ENABLE_PREWARM_WEBPROCESS_ON_PROVISIONAL_LOAD 1
+#endif

Source/WebKit/UIProcess/WebPageProxy.cpp

@@ -5389,6 +5389,17 @@ void WebPageProxy::didStartProvisionalLoadForFrame(FrameIdentifier frameID, Fram
     didStartProvisionalLoadForFrameShared(m_process.copyRef(), frameID, WTFMove(frameInfo), WTFMove(request), navigationID, WTFMove(url), WTFMove(unreachableURL), userData);
 }
 
+static bool shouldPrewarmWebProcessOnProvisionalLoad()
+{
+#if ENABLE(PREWARM_WEBPROCESS_ON_PROVISIONAL_LOAD)
+    // With sufficient number of cores, page load times improve when prewarming a Web process when the provisional load starts.
+    // Otherwise, a Web process will be prewarmed when the main frame load is finished.
+    return WTF::numberOfProcessorCores() > 4;
+#else
+    return false;
+#endif
+}
+
 void WebPageProxy::didStartProvisionalLoadForFrameShared(Ref<WebProcessProxy>&& process, FrameIdentifier frameID, FrameInfoData&& frameInfo, ResourceRequest&& request, uint64_t navigationID, URL&& url, URL&& unreachableURL, const UserData& userData)
 {
     PageClientProtector protector(pageClient());
@@ -5416,7 +5427,8 @@ void WebPageProxy::didStartProvisionalLoadForFrameShared(Ref<WebProcessProxy>&&
     internals().pageLoadState.clearPendingAPIRequest(transaction);
 
     if (frame->isMainFrame()) {
-        notifyProcessPoolToPrewarm();
+        if (shouldPrewarmWebProcessOnProvisionalLoad())
+            notifyProcessPoolToPrewarm();
         process->didStartProvisionalLoadForMainFrame(url);
         reportPageLoadResult(ResourceError { ResourceError::Type::Cancellation });
         internals().pageLoadStart = MonotonicTime::now();
@@ -5935,6 +5947,9 @@ void WebPageProxy::didFinishLoadForFrame(FrameIdentifier frameID, FrameInfoData&
 
         resetRecentCrashCountSoon();
 
+        if (!shouldPrewarmWebProcessOnProvisionalLoad())
+            notifyProcessPoolToPrewarm();
+
         callLoadCompletionHandlersIfNecessary(true);
     }