I did a memory usage test of Windows 7 64 bit with 4 GB RAM. Since Windows 7 takes advantage of more RAM, let’s see the result if we test with 8 GB RAM. If you are using 8 GB RAM for your VDI sizing, this test will be more applicable than the test with 4 GB RAM because Windows 7 will make use of the extra RAM. It is smart enough to utilised the hardware since it’s already given to it.
BTW, if what you want is Windows 2008, see this.
Both tests (4 GB and 8 GB) were conducted on physical desktops. I did it on physical so we know for sure there is no hypervisor impacting any reading here. I know VMware VMkernel will not impact the reading, but to assure some readers, I decided to eliminate that layer altogether. The 2 desktops are not identical but they have the same set of applications. The 8 GB desktop drives a 4K monitor, while the 4 GB desktop drives a Full HD monitor. I’m not sure if the 4K display impacts RAM, as I thought it uses the video RAM instead. One thing for sure, it’s much easier to review on 4K display!
In my system, since I used SSD, SuperFetch is disabled. This informative post provides details on SuperFetch. BTW, if you are using VDI, the Horizon 6 manual recommends disabling it. The reason is “By disabling the Windows prefetch and superfetch features, you can avoid generating prefetch files and the overhead associated with prefetch and superfetch operations. This action can reduce the growth of linked-clone machines and minimize IOPS on full virtual machines and linked clones”
With that, let’s dive to the test.
- I boot up the machine, and let it idle for a few minutes to ensure all start up programs have finished running. I want them to “settle down”…
- Windows 7 makes use of all the physical RAM. The 8 GB desktop takes 1 GB more RAM compared with the 4 GB desktop. I do not have any applications running and have listed all the processes shown in Task Manager. Windows 7 takes up 3 GB right away.
- I performed a similar test to the one I did on the 4 GB desktop. Essentially, I launched a lot of common apps, and opened lots of large files (>10 MB on average). For the video, I opened a 650 MB video.
- I also forced PowerPoint to load all the slides, by going into slide sorter and made it draw all slides.
- Naturally, CPU and Disk would spike, so I let them settle down first. My focus here is RAM.
- The screenshot is taken after Windows 7 settles down. As you can see, CPU metrics have gone down for all processes. PowerPoint, Visio, Adobe, Word, etc. have gone down to 0%, as they are done opening files.
- Surprisingly, Windows 7 still have 1.5 GB of free RAM. This tells me that 6.5 GB is comfortable.
- Just in case you think 8 GB RAM is too much, you can easily hit it by opening more applications and files. In the screenshot above, the Free memory dropped to 61 MB. It actually touched 0 MB for a second. Windows did not seem to like the 0 value and would move some pages out.
- The screenshot was again taken after CPU and Disk stabilised. The RAM is also stable around 6 GB. Windows “Used Physical Memory” graph at the top right does not take into account Standby Memory.
- Should you take into account Standby? That seems debatable.
- There is another way to know if Windows needs more RAM. I recommend you read this excellent article by Ed Bott. The link to the screenshots is not working, which is one reason I recreated my own.
- From the PerfMon, I can see that my Commit Limit is 16 GB. Commit Limit = Physical RAM + Virtual RAM. In my case, that’s 8 GB of RAM and 8 GB of PageFile.sys. So 16 GB is all I have.
- % Committed is what is currently committed / Commit Limit. In my case, Windows commits 6.2 GB. The value is stable, so the % Committed is stable at 38%.
- Should you use your Virtual RAM? There are different opinions on the Internet. I personally prefer what I recommend at the end of this blog.
- BTW, the pagefile.sys is typically located in C:\ directory. It’s hidden, just like the file for hibernation. By default, Windows 7 automatically manages the pagefile. I notice the same behaviour in Windows 8.1. Both basically creates a pagefile the size of your physical RAM. Whether pagefile is good or bad, that is again debatable…. My experience is at 8 GB, you need it. Windows 7 performs poorer without it.
- One thing for sure. Pagefile is not swap file. Swap file is used when Windows runs out of physical RAM. Pagefile is used proactively. Just because you’re seeing activity in pagefile.sys does not mean Windows is running out of RAM.
- To complete the test, I closed all applications. You can see in the top right corner, the chart changes in value in tandem as Windows closed the applications.
- Interestingly, the usage did not go back to original. Windows is using ~4.6 GB of RAM. I think some applications do not actually leave the RAM. Skype, Chrome are examples of such applications. I have seen Chrome taking up >1 GB of RAM if you let it run for days.
- BTW, Windows 7 keeps the pages in the Standby memory. It does not move it out after a few minutes. To me, this makes engineering sense. It is the same strategy adopted by ESXi VMkernel, which is why you see the Memory Consumed counter to be high.
Screenshot #6 (video)
- I’m curious the impact of playing video on RAM. I played a 650 MB video. Surprisingly, it did not occupy 650 MB. In fact, it occupied only 360 MB. How I know is the Free memory dropped by 360 MB. I played the video in full size (1:1), not full screen, which explains why it did not occupy the full screen as it’s 4K display.
- During the play, the memory counter did not slowly go up. It remains essentially the same, as you can see above.
- I thought perhaps because I was simply watching the video normally (sequentially). So I jumped along the video, forcing it to play randomly. I would click toward the end, let it play, then immediately click somewhere at the beginning. It is a 27 minutes video, so I have plenty of timeline to click. It’s interesting to see that this random jump does not result in page fault. It is as if the entire video is already in memory. The counter Hard Faults/sec barely moved. Perhaps it was reading from disk directly?
- If you want the best performance, use Total – Free as the sizing.
- If you want a more cost effective solution, use Total – Free – Standby. This would result in around 1-3 GB less RAM.
- Let Windows manage the pagefile. This is the default setting anyway. I noticed a visibly slower performance even though Windows showing >1 GB of Free memory. In fact, Windows gave error message, and some applications crashed.
- The % Committed metric should not hit 80%. Performance drops when it hits 90%, as if it’s a hard threshold used by Windows. If you use a pagefile, you will not hit this limit.
- In general, I’d size Windows 7 between 4 – 8 GB of RAM, depending on the users. I’d use the following guidelines
- 4 GB for light user
- 8 GB for average
- 12 GB for heavy. Yes, that’s 12 GB as I’ve seen my customers hit near 0 Free Memory, and he is just a “normal, average user”. He is an IT Manager.
You might be wondering what those memory counters mean in Task Manager. Naveed Qadri explains it well here, so please read it.
- Cached = Standby + Modified
- Available = Standby + Free
- Free = Free + Zero.
The difference between Cached and Available is Cached uses Modified, while Available uses Free. Available means exactly what the word means. It is the amount of physical memory immediately available for used.
Another article on Windows 7 memory management that I found useful was one written by Brandon Paddock. You can find it here. He wrote a program that proves that Commit can go up without In Use going up. I’m going to quote a sentence, so you can find it in his blog.
“Notice how my physical memory usage is unchanged, despite the fact that Commit has now increased by the full 2.3GB of that file."
Committed RAM can go beyond the physical RAM, as it takes into account pagefile.sys. The Commit Limit is typically 2x your physical RAM. In the example that Brandon gave:
"In fact, my commit value is now 6GB, even though I have only 4GB of physical memory and less than 3GB in use.”
Mark Russinovich explains in this technet blog something that you need to know. There is Reserved memory, and then there is committed memory. Some applications like to have its committed memory in 1 long contiguous block, so it reserves a large chunk up front. I can think of Databases and JVM in this example. This reserved memory does not actually store meaningful application data or executable. Only when the application commits the page that it becomes used. Mark explains that “when a process commits a region of virtual memory, the OS guarantees that it can maintain all the data the process stores in the memory either in physical memory or on disk”.
Notice the word “on disk“. Yes, that’s where the pagefile.sys comes in. Windows will use either the physical memory or the pagefile.sys.