Why doesn't an SSD improve the speed of running applications compared to a HDD?

Question

In articles/forums about PC gaming I always see people saying how Solid State Drives only make games load faster at startup (of the application) than they would with a Hard Disk Drive, and that an SSD doesn't improve the speed/performance of an application that is executing (after startup) in comparison to a HDD. Now I imagine that the first part of this is because during application startup, the operating system needs to fetch all of the application's instructions from the disk, and load them into RAM. And since SSD access is a lot faster than HDD access, program startup with a SSD should be a lot faster than a HDD.

Now what I don't understand is how this doesn't also affect programs that are currently running, since the OS can move a running application's memory pages from RAM to the page file/swap space on the disk. When the running program needs to access a page that was moved to the page file/swap space, a disk read needs to occur in order for the page to be brought back into RAM to be used by the application. So wouldn't an SSD also provide a performance increase here too, as opposed to just in application startup?

Answer 1

You ask why SSDs don’t necessarily improve the speed of running applications compared to a traditional HDD. The reason is disk access. SSDs do read and write much faster than traditional hard disks. However, unless the application is heavy on disk I/O (reading and writing to the disk), the benefits of the SSD are lost. Even in cases where there is some writing to disk, the OS has a disk cache. This means writes are written to RAM and eventually flushed to the disk at a later point. The disk cache significantly improves the speed when using traditional HDDs.

As for swapping, yes, SSDs will perform faster, but you probably won’t notice the difference. Unless you are severely starved for free memory and the OS is heavily swapping, you will not notice the speed difference.

Answer 2

Generally swapping occurs only when you start running out of RAM. With today's systems with 16 GB and higher, it is unlikely that playing games would cause swapping to occur. If swapping does occur, then yes, it would be much faster when using an SSD than a HDD. However, the page file is often disabled on SSDs to limit the number of writes and thus increase lifespan a little bit.

See: Should the pagefile be placed on the SSD or the HDD?  The exception to the "swapping only when low on RAM" rule is explained in this question: Why does Windows 7 use the page file when there’s free physical RAM?

Answer 3

A. When your system starts to swap, you are already in trouble. You don't want to get there, ever.

B. SSDs are faster than conventional hard disks, but they are still significantly slower than RAM memory.

A running game in an adequately designed system uses only CPU, GPU, and RAM, and the hard disk is accessed only for saving game status or such. Therefore, an SSD cannot make it faster, except if the system is already overextended severely (and then more memory will do more good).

Answer 4

That's simple. Any system always has a single bottleneck; its weakest element that limits overall performance. Any improvement outside of the bottleneck is useless (and sometimes harmful).

If switching to SSD doesn't improve the performance of your application (game etc.), then disk I/O is not its bottleneck.

To find the right place to improve, monitor the resources in the runtime of your application. What is used close to 100%? Is it CPU, GPU, RAM, disk I/O? Does your machine use swap? If it does, extra RAM can be more useful than SSD.

Answer 5

This is actually quite an interesting question. To answer it, I need to give you a brief over view on how games are typically written these days.

But yes, the article is sort of right. But it is much more complicated than it would appear.

Firstly. What everybody has talked about regarding normal applications is correct. Swap drive usage will severely degradate performance.

Secondly. A SSD will improve gaming performance on a machine that has more than enough RAM to avoid traditional swapping.

Thirdly. That performance boost will be seen ONLY during "level loading".

The reason for this happening is because of the 3D Graphics Engine. Typical 3D games need to draw highly detailed 3D models constantly. In order to do so, the 3D engine will have to hold many GBs of textures in the video card.

Game developers, in a bid to make more complicated and detailed visuals, have been steadily increasing the size of the textures in each game. In fact, the VAST majority of a modern game is going to be made up of textures.

You will notice most games will not fit into your video card's memory. For example, my GPU has 2 GB of VRAM, whereas a modern game like Overwatch takes up something like 6 GB of storage.

To bridge this discrepancy, modern video games will swap textures in and out of the HDD/SSD as needed. Unfortunately this process is quite disruptive. Which is why it is usually done during "level loading".