Как добавление памяти значительно увеличило производительность игр для AMD APU?

To be clear from the start, this is not a problem. This is just something that I really want know the secret behind.

System Specs

• CPU: AMD A10-6790K 4.0 GHz
• GPU: AMD Radeon HD 8670D 1GB (integrated GPU)
• RAM: 2 x Team 4GB 1600 DDR3 = 8 GB

Of course, I am using a 64-bit OS to make use of my 8 GB of memory, but my question is: Before I installed the additional 4 GB of RAM, games like Mafia II ran (at the highest settings) at an average of 22 FPS. After I installed the additional RAM, I noticed a very decent increase to 40 FPS even though the game did not appear to use more than 4 GB of RAM.

What is the secret behind this?

I noticed you are using an AMD APU. These chips combine a CPU with a built-in graphics processor (GPU), eliminating the need for a discrete graphics card (at least for lighter workloads). Because AMD APUs use the system memory as graphics memory, integrated GPU performance is highly dependent on memory bandwidth. Not only does DDR3 system RAM have considerably less bandwidth than GDDR5 video memory (used on many discrete graphics cards), the integrated GPU needs to share this bandwidth with the CPU for normal application use. Increasing memory bandwidth will directly increase performance by reducing this bottleneck.

When you upgraded your system’s memory, you added a second memory module. With two modules, your memory now runs in dual-channel mode, doubling memory bandwidth and dramatically increasing performance as a result. Faster RAM (at least DDR3-1866, preferably DDR3-2100+) will similarly increase performance as well.

In addition, more memory means your system can pre-load more texture data into RAM, reducing the need to access the disk and increasing performance. However, this is less of a factor than the increased memory bandwidth.

To show how important memory bandwidth is for AMD APUs, websites such as Tom’s Hardware and PC Perspective have found that APU performance can increase with memory bandwidth up to at least DDR3-2100.

You have an APU, rather than a discrete CPU and discrete GPU. That means they share system RAM for texture cache, rather than dedicated on-board memory on a graphics card.

The reason the RAM upgrade sped things up for your game is likely because of texture resource swapping. With more RAM available overall, that means more texture data is able to be loaded and kept in RAM longer. When you have all your memory allocated, and your operating system needs more to work with for a task right now, it will find allocated memory that has not been used recently, save the contents to your disk, and then re-allocate the memory for that task. Once that data is needed again, it will swap it back from the disk into RAM. This swapping takes a long time (relatively).

You were likely swapping a lot on texture data. When you upgraded your RAM, you provided more space for textures to be stored, which means less memory swapping, which then translates into a higher FPS.