Safari is now integrated with Twitter and displays tweets from people you follow in the new Sidebar Shared Links section on the left.
Apple engineers managed to build in some performance boosts; pages do tend to display a little more snappily. Safari is also more efficient. The new Safari Power Saver can shut down webpages in the background if, for instance, those pages have Flash- or HTML 5-based ads or looping animations. That should offer a small boost to battery life if you're using a laptop, since you're not needlessly using CPU cycles for something you're not even viewing.
Notifications, which first appeared in last year's release of Mountain Lion, have been updated with additional functions. For instance, if an instant message comes in, you can reply directly from the notification itself when it pops up on the screen. That's handy if the main Messages window is hidden behind another window or in another desktop Space.
Notifications can be displayed in front of your screen saver if you have one activated, similar to how notifications can be displayed on the Lock Screen of an iOS device. Notification bubbles float on the upper right of your screen, each displaying varying levels of information, depending on the message and your display settings. (You can tweak these settings in System Preferences > Notifications.)
Websites can now send notifications, too, even if Safari isn't running. That's useful if you want updates from places like Facebook or a news site you follow. But notifications don't always show up consistently. Sometimes they're instant, sometimes they're not. And sometimes an instant message will show up in a Messages chat window, and then later I'll get a notification of the same message. This is something Apple needs to sort out if it expects Mavericks users to rely on notifications.
Under the hood
Mavericks introduces several under-the-hood changes (PDF) designed to make the OS more efficient. Apple engineers found a way to decrease overall memory use by compressing any memory that hasn't been used, reducing the need for virtual memory. The compression and decompression happens in a few millionths of a second, according to Apple, utilizing multiple CPU cores to perform the tasks. This should lead to better battery life on laptops by reducing swap-file writes to the hard drive.
Apple has also rolled out new technology called Timer Coalescing, which bunches background operations into like-timed groups and then processes these operations together. This allows the CPU to remain idle for longer intervals, again leading to incrementally better battery life. This is a much better strategy than constantly firing up the CPU every time a process needs to run.
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