For Laravel 5.0.27. Improvement to prevent writing session data even if it has not changed. Improvement to specify session metadata update frequency in session configuration instead of updating on every request. These both improvements will also reduce the risk for race conditions for concurrent requests.

session_write_improvements.patch

Improvement to prevent writing session data even if it has not changed. 
Improvement to specify session metadata update frequency in session configuration instead of updating on every request.
These both improvements will also reduce the risk for race conditions for concurrent requests.
Also, add 'metadata_update_threshold' => 1, to your config/session.php

diff --git a/vendor/laravel/framework/src/Illuminate/Session/EncryptedStore.php b/vendor/laravel/framework/src/Illuminate/Session/EncryptedStore.php
index edf34b8..0b5b763 100644
--- a/vendor/laravel/framework/src/Illuminate/Session/EncryptedStore.php
+++ b/vendor/laravel/framework/src/Illuminate/Session/EncryptedStore.php
@@ -20,13 +20,14 @@
 	 * @param  \SessionHandlerInterface $handler
 	 * @param  \Illuminate\Contracts\Encryption\Encrypter $encrypter
 	 * @param  string|null $id
+     * @param  int|0 $metaUpdateThreshold
 	 * @return void
 	 */
-	public function __construct($name, SessionHandlerInterface $handler, EncrypterContract $encrypter, $id = null)
+	public function __construct($name, SessionHandlerInterface $handler, EncrypterContract $encrypter, $id = null, $metaUpdateThreshold = 0)
 	{
 		$this->encrypter = $encrypter;
 
-		parent::__construct($name, $handler, $id);
+		parent::__construct($name, $handler, $id, $metaUpdateThreshold);
 
 	}
 
diff --git a/vendor/laravel/framework/src/Illuminate/Session/SessionManager.php b/vendor/laravel/framework/src/Illuminate/Session/SessionManager.php
index b231598..d2167b2 100644
--- a/vendor/laravel/framework/src/Illuminate/Session/SessionManager.php
+++ b/vendor/laravel/framework/src/Illuminate/Session/SessionManager.php
@@ -162,15 +162,18 @@
 	 */
 	protected function buildSession($handler)
 	{
+		$metaUpdateThreshold =
+			$this->app['config']['session.metadata_update_threshold'] * 60;
+
 		if ($this->app['config']['session.encrypt'])
 		{
 			return new EncryptedStore(
-				$this->app['config']['session.cookie'], $handler, $this->app['encrypter']
+				$this->app['config']['session.cookie'], $handler, $this->app['encrypter'], null, $metaUpdateThreshold
 			);
 		}
 		else
 		{
-			return new Store($this->app['config']['session.cookie'], $handler);
+			return new Store($this->app['config']['session.cookie'], $handler, null, $metaUpdateThreshold);
 		}
 	}
 
diff --git a/vendor/laravel/framework/src/Illuminate/Session/Store.php b/vendor/laravel/framework/src/Illuminate/Session/Store.php
index 41a3593..ffbfa6a 100644
--- a/vendor/laravel/framework/src/Illuminate/Session/Store.php
+++ b/vendor/laravel/framework/src/Illuminate/Session/Store.php
@@ -29,6 +29,13 @@
 	 */
 	protected $attributes = array();
 
+	/**
+	 * Raw session data to detect changes before storing.
+	 *
+	 * @var string
+	*/
+    protected $rawInitialData;
+
 	/**
 	 * The session bags.
 	 *
@@ -70,14 +77,15 @@
 	 * @param  string $name
 	 * @param  \SessionHandlerInterface $handler
 	 * @param  string|null $id
+	 * @param  int|0 $metaUpdateThreshold
 	 * @return void
 	 */
-	public function __construct($name, SessionHandlerInterface $handler, $id = null)
+	public function __construct($name, SessionHandlerInterface $handler, $id = null, $metaUpdateThreshold = 0)
 	{
 		$this->setId($id);
 		$this->name = $name;
 		$this->handler = $handler;
-		$this->metaBag = new MetadataBag;
+		$this->metaBag = new MetadataBag('_sf2_meta', $metaUpdateThreshold);
 	}
 
 	/**
@@ -117,10 +125,13 @@
 	protected function readFromHandler()
 	{
 		$data = $this->handler->read($this->getId());
-
+
 		if ($data)
-		{
-			$data = @unserialize($this->prepareForUnserialize($data));
+		{
+			$data = $this->prepareForUnserialize($data);
+			$this->rawInitialData = $data;
+
+			$data = @unserialize($data);
 
 			if ($data !== false) return $data;
 		}
@@ -252,8 +263,12 @@
 
 		$this->ageFlashData();
 
-		$this->handler->write($this->getId(), $this->prepareForStorage(serialize($this->attributes)));
-
+		$serialized = serialize($this->attributes);
+
+		if($serialized !== $this->rawInitialData){
+			$this->handler->write($this->getId(), $this->prepareForStorage($serialized));
+		}
+
 		$this->started = false;
 	}
 

Make sure you add

'metadata_update_threshold' => 1,

to your config/session.php for this patch to become effective.

The patch should be applied to vendor folder each time it gets recreated.