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Many of my friends have said that installing multiple operating systems on a single PC slows down individual OS's because the total RAM gets divided among the OS's.

Is there any truth in the above statement?

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Ignacio is 100% correct. Your friends are either just plain wrong, confusing RAM with HDD space or presuming virtualization as apposed to a multi-boot scenario... my bet is they are just wrong. –  Windos Aug 10 '11 at 5:36
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Not unless you try running both of them at once. –  Shadur Aug 10 '11 at 6:42
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i agree. This sounds like the friends were talking about running a virtual guest o/s at the same time, not dual booting. Just a misunderstanding i think. –  Sirex Aug 10 '11 at 6:53
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This is along the same line of the lie many people believe about the number of files on their computer contributing to system slowness. Or the even more silly, the number of icons on their desktop correlating to system slowness. Both these cases are untrue, as is this one. –  music2myear Aug 10 '11 at 18:03
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@music2myear: when you start talking about correlation, you need to be careful... there could be confounding variables (e.g. people who don't clean up their desktop also install loads of programs which run silently in the background) ... ;-) –  Jonas Heidelberg Aug 10 '11 at 18:46
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11 Answers

up vote 67 down vote accepted

It depends on how you install the multiple operating systems – there are generally two options

  • installing for dual boot
  • installing using a VM

Dual boot installation just places the other OS on free space on your hard disk, so it will use hard disk space (you may need/be-asked-to to create new partitions), but since in a dual boot only one OS will run at any given time, then no memory or CPU is being used by the other OS.

Virtual machine (VM) is a software program which emulates the hardware of a PC. If you use a VM to install your second OS, then both OS can run at the same time – so you can have a Windows PC which has Linux running in a VM-window, and you can use both at the same time. Since you can use both at the same time, they both will use memory and CPU, and then you may see the computer slowing down.

If you don't know anything about how to use a VM, then it is unlikely that you have one, but rather that you have a dual boot system, in which case – NO, you will not see the system slowing down.

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A more complete answer than mine. but I would quibble with the "both OS can run/use at the same time". There is a host OS that is allowing the guest OS to execute as a program with special privileges and capabilities. The guest OS (and any applications it runs) can be viewed as a privileged application program still under the purview of the multiprocessing OS that booted the PC. –  sawdust Aug 10 '11 at 6:11
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@sawdust - Not entirely true. What if you are running a bare-metal hypervisor, like Xen? –  Fake Name Aug 10 '11 at 7:06
    
These days with 7's xp mode, people do sometimes use virtualization without really knowing it. –  Icode4food Aug 10 '11 at 12:41
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+1: I think this is the best answer, because it explains virtualization (rather than just obliquely referring to it and expecting that the questioner already understands it). –  Ken Bloom Aug 10 '11 at 13:49
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@Fake Name -- good point on that not all hypervisors implement the machine in software (bare-metal), however the basis of the quetion of does the machine slow down is un affected -- ram, cpu, disk and network all becomes shared resources regardless, and hence the machine performance IS affected. –  Soren Aug 10 '11 at 21:23
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The OS you are running will not slow down. Only the hard disk capacity will be decreased.

Theoretically your OS may slow down if you have a running program in the background which scans files on hard disk, because this program will scan all files from all operating system. But this is just a theoretical slowdown which you will not notice.

If you plan to have different OSs in your hard disk (Windows, Linux, MacOS), then Windows OS will not see them and will not be slowed down at all.

You may also consider having virtual OSs via VirtualBox. Myself, for example, have more than 10 OSs and 1 main OS. When I need, let's say, Win 7 for design, I start its virtual machine and have one system running inside other system. I have 8 GB of RAM and I give 4GB RAM to each system and the virtual OS is not slow at all. Just to mention that these virtual OSs are not suitable for gaming.

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A disk scanning program would only scan hard drives that have been mounted. So if you run Linux/Windows then it's unlikely that Windows will be slowed down (as it can't mount ext type filesystems) and Linux distros usually won't mount the NTFS drive by default. –  Jonathan Sternberg Aug 10 '11 at 19:31
    
You are right about Windows, but Linux like Ubuntu offer automount on install so for the last 4 years every Ubuntu distro I use DOES automount all windows partitions. ;) –  JoeM Aug 11 '11 at 7:45
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No: As long as the OS' are installed on dedicated partitions/disks they will not affect the speed of each other.

Yes: There are factors that will cause an OS to go slower. If the partition contains many files (say you install both OS on the same partition (really not recommended as files will conflict)). Another more likely factor is that the first partition (and the first file) is stored on the outmost rings of the disk, so the HD head needs to move less (bigger cirles, more data per cirle). Any files/partition further in will get increasing head moves vs bytes read. So if the second operating system is for example 50% out on the disk the speed will be slightly lower. But it should be said that due to various reasons such as cylinder scew and caching this is not noticeable.

So the answer is: No. The operating systems will not be noticeable slower.

As others have already pointed out running OS as virtual guests will slow down both host and guest OS.

Best advice; Make sure you have plenty of RAM in host OS. The lack of plenty of RAM makes your computer SLOW. I myself run Win7 with 8GB (desktop) and 12GB (laptop) and I've just placed an order for 16GB for my desktop. That is a bit over the top for most users, but benificial for my use.

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Installing more than one OS will not slow your computer because they are stored on the hard disk. When you turn on the PC only one OS will be running at a given time.

The computer speed may be slow due to shortage of RAM. In my opinion if you have 2GB of RAM then you can run any OS smoothly.

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When it's dual-boot, you run only one system at the time, so it wouldn't slowdown for you. Only "way" to slowdown it to run another system on virtual machine

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if you are using virtual OS then your PC will decrease its performance but if you used dual boot system then it will work normally.

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It possibly can slow down if:

  1. You don't have enough memory in your PC

  2. The OS has to depend on paging and store memory data on your hard drive

In this case installing an additional OS would not slow the PC down, but as your hard drive gets more data and becomes fragmented, it would start to slow down.

Make sure you have enough disk space and RAM.

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For the most part, no, installing multiple operating systems will not slow down the computer, unless you are using virtualization to run two or more at the same time.

However, there is one thing that will slow down when using a standard hard disk. File access to operating system files.

In almost all mechanical hard drives the early parts of the disk are the fastest parts. Almost all operating systems install their boot files here and many advanced defrag programs for Windows will move the most accessed files to the first parts of the disk.

Installing multiple operating systems means that some of the operating systems will be installed onto slower parts of the disk.

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That depends on how it's running all the systems. Are they running in a VM or in a dual boot? In the case of a dual boot, no this is not true at all. For a VM however, resources must be allocated to the VM by the host OS. Think of Java for example. Although not an OS, it runs in a VM and runs slower (although is probably more due to having to interpret code).

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No.the speed of computer does not decrease until and unless you are running the two O.S. at same time. Actually when you select one O.S. choice among the two or more at start-up then the required system files of that O.S. will run on RAM and you see only the selected O.S. running. Installing two or more O.S. will not cause slow down the speed of O.S. but consume the space on hard disk.

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If you try to run the operating system in VM for sure it will slow down, but if you install separately on different partition there is no problem.

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protected by Gilles Aug 21 '11 at 20:54

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