Windows 10 uses memory compression to store more data in your system's memory. If you visit Task Manager and examine the details of using your memory, then you will probably see that some of your memory is "compressed". Let's see what that means.
What is memory compression?
Memory compression is a new feature of Windows 10, not available in Windows 7 and 8. However, Apple's Linux and MacOS also use memory compression.
Traditionally, if you had 8 GB RAM and the applications as a whole had 9 GB data to store in this RAM, at least 1 GB would have to be paged out and stored in the pagefile on your computer's disk. Access to the pagefile data is very slow compared to RAM.
By compressing memory, some data from 9 GB can be compressed (just like a compressed file or other compressed data can be shrunk by consuming less space) and kept in RAM. For example, you may have 6 GB of uncompressed data and 3 GB files that if compressed will become 1,5 GB in RAM. If you would like to store all the 9 GB of the original data on your 8 GB RAM, you could use the partial compression method as you would only need 7,5 GB.
Is this method disadvantageous? Well, yes and no. Data compression and decompression requires some CPU resources, which is why not all data is stored in compressed form. Compression of RAM occurs only when Windows thinks it is necessary and useful. Compressing and decompressing the data at some CPU moment is a matter, but it is faster than storing and reading the data in the pagefile on the system's hard disk. So there is always a dilemma trying to solve Windows.
Is the Memory Squeeze Problem?
Data compression in memory is much better than the alternative, to store the data in the disk. It's faster than using the pagefile. Windows will automatically squeeze the data into memory when space is needed.
But compression memory is using some CPU resources. Your system can not perform tasks as fast as it would if it did not need to compress data into memory. If you see a large amount of compressed memory and you suspect that is why your computer is a bit slow, the only solution for this is to install more physical memory (RAM) on your system. If your computer does not have enough physical memory for the applications you use, memory compression is better than using the pagefile, but most of the physical memory is always the best solution.
How to View Compressed Memory Details on Your Computer
To see how much memory is being compressed on your system, you need to use Task Manager. To open it, either right-click on the taskbar and select Task Manager, or press Ctrl + Shift + Esc, or press Ctrl + Alt + Delete and click Task Manager.
If you see the Simple Task Manager interface, click 'More Details' at the bottom of the window.
Click the 'Performance' tab and select 'Memory'. You will see how much memory is compressed down to "In Use (Compressed)". For example, in the screenshot below, Task Manager shows that our system is currently using 2,7 GB of its physical memory. The 254 MB from it is compressed memory and represents 645 MB which would otherwise be uncompressed and would occupy the RAM. This saves 391 MB memory.
You will see a number that fluctuates over time as they open and close applications. There are also fluctuations as system services run in the background, and you can see if you monitor the task manager window for a minute.
If you put your mouse over the leftmost part of the memory bar, you will see more details about your compressed memory.
Does this make the system process use a lot of memory?
In the original version of Windows 10, the "compression store" was stored in system processes and that was "the reason the System process seemed to consume more memory than previous versions", according to Microsoft blog.
However, at some point, Microsoft changed the way it works. Compressed memory no longer appears as part of the System process in Task Manager (perhaps because it was very confusing to users). Instead, it is visible under the "Performance" tab in the "Memory" item.
In Windows Creators Update, the compressed memory is only displayed in Memory, and the System process remains at low levels even when the system has a lot of compressed memory. This saves confusion as users will not be wondering why the process is mysteriously using so much memory.