fs: improve readFile performance

This increases the maximum buffer size per read to 512kb when using
`fs.readFile`. This is important to improve the read performance for
bigger files.

PR-URL: #27063
Refs: #25741
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jamie Davis <davisjam@vt.edu>

lib/fs.js

@@ -266,19 +266,17 @@ function readFileAfterStat(err, stats) {
 
   const size = context.size = isFileType(stats, S_IFREG) ? stats[8] : 0;
 
-  if (size === 0) {
-    context.buffers = [];
-    context.read();
-    return;
-  }
-
   if (size > kMaxLength) {
     err = new ERR_FS_FILE_TOO_LARGE(size);
     return context.close(err);
   }
 
   try {
-    context.buffer = Buffer.allocUnsafeSlow(size);
+    if (size === 0) {
+      context.buffers = [];
+    } else {
+      context.buffer = Buffer.allocUnsafeSlow(size);
+    }
   } catch (err) {
     return context.close(err);
   }

lib/internal/fs/read_file_context.js

@@ -6,27 +6,32 @@ const { Buffer } = require('buffer');
 
 const { FSReqCallback, close, read } = internalBinding('fs');
 
-const kReadFileBufferLength = 8 * 1024;
+// Use 64kb in case the file type is not a regular file and thus do not know the
+// actual file size. Increasing the value further results in more frequent over
+// allocation for small files and consumes CPU time and memory that should be
+// used else wise.
+// Use up to 512kb per read otherwise to partition reading big files to prevent
+// blocking other threads in case the available threads are all in use.
+const kReadFileUnknownBufferLength = 64 * 1024;
+const kReadFileBufferLength = 512 * 1024;
 
 function readFileAfterRead(err, bytesRead) {
   const context = this.context;
 
   if (err)
     return context.close(err);
 
-  if (bytesRead === 0)
-    return context.close();
-
   context.pos += bytesRead;
 
-  if (context.size !== 0) {
-    if (context.pos === context.size)
-      context.close();
-    else
-      context.read();
+  if (context.pos === context.size || bytesRead === 0) {
+    context.close();
   } else {
-    // Unknown size, just read until we don't get bytes.
-    context.buffers.push(context.buffer.slice(0, bytesRead));
+    if (context.size === 0) {
+      // Unknown size, just read until we don't get bytes.
+      const buffer = bytesRead === kReadFileUnknownBufferLength ?
+        context.buffer : context.buffer.slice(0, bytesRead);
+      context.buffers.push(buffer);
+    }
     context.read();
   }
 }
@@ -60,7 +65,7 @@ class ReadFileContext {
   constructor(callback, encoding) {
     this.fd = undefined;
     this.isUserFd = undefined;
-    this.size = undefined;
+    this.size = 0;
     this.callback = callback;
     this.buffers = null;
     this.buffer = null;
@@ -75,9 +80,10 @@ class ReadFileContext {
     let length;
 
     if (this.size === 0) {
-      buffer = this.buffer = Buffer.allocUnsafeSlow(kReadFileBufferLength);
+      buffer = Buffer.allocUnsafeSlow(kReadFileUnknownBufferLength);
       offset = 0;
-      length = kReadFileBufferLength;
+      length = kReadFileUnknownBufferLength;
+      this.buffer = buffer;
     } else {
       buffer = this.buffer;
       offset = this.pos;