Mission Control is one of the major features of Lion, and to take full advantage of it, you need all your frequently used applications open. Apple is also encouraging users of Macs to use their computers much like iOS devices, i.e. just concentrating on the task at hand and allowing the OS to handle memory management and application closing etc.
While this works almost flawlessly in iOS devices due to their flash storage and small application sizes, in Macs, this could lead to system slow-downs and increased power consumption.
So what causes the problem?
Let’s say you have 4GB of system memory, and your open applications use up 3.5GB leaving 0.5GB left for whatever you want to do next. Now let’s say that we load a Photoshop file into the memory which uses 1GB. With only 0.5GB of memory left, the OS must find a way of freeing up an extra 0.5GB of memory for the Photoshop file. It does this by paging.
Any item in the memory which is not being used is moved from the memory onto the “virtual memory” on the hard drive. This frees up enough space for the 1GB photoshop file we want to open.
The problem with paging is that it is incredibly slow. The hard drive is the slowest component in the system, and relying on this for paging would result in slowdowns as the hard drive must a) load the file into the memory and b) write data into the virtual memory.
Why aren’t the MacBook Airs affected so much?
The 2010/2011 MacBook Airs use SSD storage (Solid State Drive). SSD’s have no moving parts and are much faster than hard drives found in most MacBook Pros; therefore paging into an SSD would be much faster than paging onto a hard drive. This is partly the reason for why the base 11 inch MacBook Air (2011) with only 2GB of RAM still performs at an acceptable level, because paging onto an SSD is fast and results in slightly less slowdown. RAM is still faster than an SSD though, so the 4GB Air would still perform faster than its 2GB counterpart.
So how much is enough?
In the days of Snow Leopard, I would have said that 4GB would be ample RAM for any Mac under a basic workload. Lion however emphasises multitasking and takes control over closing applications and memory management, and 4GB may not be enough. If you own a Mac with an SSD, then you can easily get by with 4GB, but for a Mac running on a hard drive, paging may drastically reduce system performance if you’re opening a large application or file. Therefore, if you’re likely to be doing heavy multitasking or working with large files, 6 or 8GB of RAM is the way to go.
Dual Channel or not to Dual Channel…
Apple’s Macbook Pro range (and indeed many Windows based notebooks) have 2 RAM slots, and both are usually occupied with two identical modules. For example, a Macbook Pro could have 2x2GB modules making 4GB. The advantage of pairing up identical RAM is that the system can take advantage of something called Dual channel, in simple terms, allowing the system to access both modules in one instance instead of one at a time (i.e. single channel).
Having RAM in dual channel can increase performance slightly (especially in graphics intensive tasks using integrated graphics), but most of the time this is costly as both RAM modules must be replaced at the same time.
The truth is though, while RAM running in dual channel offers performance benefits, they tend to be negligible. Under normal workloads, you will not be able to perceive the difference between a system running dual channel and one which is not. What you would perceive however, is the effects any extra RAM put into the system; so for example, a system not running dual channel memory with 6GB of RAM should feel faster than one running dual channel but with only 4GB of RAM. Of course, a system with 6GB of dual channel RAM (assuming such a thing were possible) would run even faster.
Here are a few of my crude benchmarks
Time to boot up and launch all frequently used applications
4GB: 5min43s
8GB: 3min21S
Time to launch iPhoto with frequently used applications running
4GB: 12s
8GB: 5s
Time to launch iPhoto Slideshow
4GB: 9s
8GB: 4s
Time to create new Safari window with iPlayer running
4GB: 6s
8GB: 3s
BioShock FPS
4GB: 33FPS
8GB: 32FPS
Does RAM solve everything?
Unfortunately not; adding more RAM essentially reduces the need for the system to use the major bottleneck component (hard drive). While it makes the system run more efficiently, it does not make it run “faster”. Tasks which heavily rely on things other than IO such as streaming HD video, playing graphics intensive games or rendering complex images would not benefit much from more RAM. This can be seen in the BioShock FPS benchmark where the frame rate stayed the same, but it has to be noted that opening and exiting to finder is much faster with more RAM.
Posted on September 14, 2011
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