[Fizz] Expose maxBoundarySize as an option

packages/react-dom/src/server/ReactDOMFizzServerBrowser.js

@@ -18,6 +18,7 @@ import {
 
 type Options = {
   signal?: AbortSignal,
+  progressiveChunkSize?: number,
 };
 
 function renderToReadableStream(
@@ -35,7 +36,11 @@ function renderToReadableStream(
   }
   return new ReadableStream({
     start(controller) {
-      request = createRequest(children, controller);
+      request = createRequest(
+        children,
+        controller,
+        options ? options.progressiveChunkSize : undefined,
+      );
       startWork(request);
     },
     pull(controller) {

packages/react-dom/src/server/ReactDOMFizzServerNode.js

@@ -21,6 +21,10 @@ function createDrainHandler(destination, request) {
   return () => startFlowing(request);
 }
 
+type Options = {
+  progressiveChunkSize?: number,
+};
+
 type Controls = {
   // Cancel any pending I/O and put anything remaining into
   // client rendered mode.
@@ -30,8 +34,13 @@ type Controls = {
 function pipeToNodeWritable(
   children: ReactNodeList,
   destination: Writable,
+  options?: Options,
 ): Controls {
-  const request = createRequest(children, destination);
+  const request = createRequest(
+    children,
+    destination,
+    options ? options.progressiveChunkSize : undefined,
+  );
   let hasStartedFlowing = false;
   startWork(request);
   return {

packages/react-noop-renderer/src/ReactNoopServer.js

@@ -210,7 +210,11 @@ const ReactNoopServer = ReactFizzServer({
   },
 });
 
-function render(children: React$Element<any>): Destination {
+type Options = {
+  progressiveChunkSize?: number,
+};
+
+function render(children: React$Element<any>, options?: Options): Destination {
   const destination: Destination = {
     root: null,
     placeholders: new Map(),
@@ -220,7 +224,11 @@ function render(children: React$Element<any>): Destination {
       ReactNoopServer.abort(request);
     },
   };
-  const request = ReactNoopServer.createRequest(children, destination);
+  const request = ReactNoopServer.createRequest(
+    children,
+    destination,
+    options ? options.progressiveChunkSize : undefined,
+  );
   ReactNoopServer.startWork(request);
   ReactNoopServer.startFlowing(request);
   return destination;

packages/react-server/src/ReactFizzServer.js

@@ -99,7 +99,7 @@ const CLOSED = 2;
 type Request = {
   +destination: Destination,
   +responseState: ResponseState,
-  +maxBoundarySize: number,
+  +progressiveChunkSize: number,
   status: 0 | 1 | 2,
   nextSegmentId: number,
   allPendingWork: number, // when it reaches zero, we can close the connection.
@@ -113,16 +113,34 @@ type Request = {
   partialBoundaries: Array<SuspenseBoundary>, // Partially completed boundaries that can flush its segments early.
 };
 
+// This is a default heuristic for how to split up the HTML content into progressive
+// loading. Our goal is to be able to display additional new content about every 500ms.
+// Faster than that is unnecessary and should be throttled on the client. It also
+// adds unnecessary overhead to do more splits. We don't know if it's a higher or lower
+// end device but higher end suffer less from the overhead than lower end does from
+// not getting small enough pieces. We error on the side of low end.
+// We base this on low end 3G speeds which is about 500kbits per second. We assume
+// that there can be a reasonable drop off from max bandwidth which leaves you with
+// as little as 80%. We can receive half of that each 500ms - at best. In practice,
+// a little bandwidth is lost to processing and contention - e.g. CSS and images that
+// are downloaded along with the main content. So we estimate about half of that to be
+// the lower end throughput. In other words, we expect that you can at least show
+// about 12.5kb of content per 500ms. Not counting starting latency for the first
+// paint.
+// 500 * 1024 / 8 * .8 * 0.5 / 2
+const DEFAULT_PROGRESSIVE_CHUNK_SIZE = 12800;
+
 export function createRequest(
   children: ReactNodeList,
   destination: Destination,
+  progressiveChunkSize: number = DEFAULT_PROGRESSIVE_CHUNK_SIZE,
 ): Request {
   const pingedWork = [];
   const abortSet: Set<SuspendedWork> = new Set();
   const request = {
     destination,
     responseState: createResponseState(),
-    maxBoundarySize: 1024,
+    progressiveChunkSize,
     status: BUFFERING,
     nextSegmentId: 0,
     allPendingWork: 0,
@@ -642,7 +660,7 @@ function flushSegment(
     flushSubtree(request, destination, segment);
 
     return writeEndSuspenseBoundary(destination);
-  } else if (boundary.byteSize > request.maxBoundarySize) {
+  } else if (boundary.byteSize > request.progressiveChunkSize) {
     // This boundary is large and will be emitted separately so that we can progressively show
     // other content. We add it to the queue during the flush because we have to ensure that
     // the parent flushes first so that there's something to inject it into.