Is virtual machine slower than the underlying physical machine?

Question

This question is quite general, but most specifically I'm interested in knowing if virtual machine running Ubuntu Enterprise Cloud will be any slower than the same physical machine without any virtualization. How much (1%, 5%, 10%)?

Did anyone measure performance difference of web server or db server (virtual VS physical)?

If it depends on configuration, let's imagine two quad core processors, 12 GB of memory and a bunch of SSD disks, running 64-bit ubuntu enterprise server. On top of that, just 1 virtual machine allowed to use all resources available.

Answer 1

The typical experience for a general purpose server workload on a bare metal\Type 1 Hypervisor is around 1-5% of CPU overhead and 5-10% Memory overhead, with some additional overhead that varies depending on overall IO load. That is pretty much consistent in my experience for modern Guest OS's running under VMware ESX\ESXi, Microsoft Hyper-V and Xen where the underlying hardware has been appropriately designed. For 64 bit Server operating systems running on hardware that supports the most current cpu hardware virtualization extensions I would expect all Type 1 hypervisors to be heading for that 1% overhead number. KVM's maturity isn't quite up to Xen (or VMware) at this point but I see no reason to think that it would be noticeably worse than them for the example you describe.

For specific use cases though the overall\aggregate "performance" of a virtual environment can exceed bare metal \ discrete servers. Here's an example of a discussion on how a VMware Clustered implentation can be faster\better\cheaper than a bare metal Oracle RAC. VMware's memory management techniques (especially transparent page sharing) can eliminate the memory overhead almost entirely if you have enough VM's that are similar enough. The important thing in all these cases is that the performance\efficiency benefits that virtualization can deliver will only be realised if you are consolidating multiple VM's onto hosts, your example (1 VM on the host) will always be slower than bare metal to some degree.

While this is all useful the real issues in terms of Server virtualization tend to be centered around management, high availability techniques and scalability. A 2-5% CPU performance margin is not as important as being able to scale efficiently to 20, 40 or however many VM's you need on each host. You can deal with the performance hit by selecting a slightly faster CPU as your baseline, or by adding more nodes in your clusters but if the host can't scale out the number of VM's it can run, or the environment is hard to manage or unreliable then its worthless from a server virtualization perspective.

Answer 2

Yes. But that is not the question. The difference is normally neglegible (1% to 5%).

Answer 3

I would point out that virtualisation can exceed physical performance in certain situations. Since the network layer is not limited to gigabit speed (even though the hardware emulation is of a specific lan card), VM's on the same server can communicate between each other at speeds beyond that of multiple phyiscal servers with average network equipment.

Answer 4

I have just upgraded to an SSD (OCZ Vertex 2) and I run my XP VM development environment on it, I am a software developer. One thing I have noticed is that when I launch a program (one big enough to take time to load), one core of the virtual CPU pegs out. This happens when loading IE also. Since the CPU pegs out, I assume the bottleneck is the CPU and not the SSD. But it seems odd, I have a feeling that if the same thing were done on a physical machine that it would load faster and my feeling is that there is some extra processing overhead VMWare is doing that is consuming CPU on disk access.

One example, I use Delphi and on a physical machine with a regular HDD it can take 20 seconds to start from a cold boot. In the VM running off an SSD, it loads in 19 seconds from a cold start. Not much difference, I bet if the SSD were on the physical machine it would load faster. Howevere I did not check the CPU usage on the physical machine, its possible the CPU were the bottleneck there as well.

But the feel of the VM is that disk access taxes the VM.

Answer 5

Obviously a virtual machine is slower than the physical machine. But when you're in this scenario you have to evaluate what is optimal to cover your needs. If you need only one system and you need it to be fast, then install it directly to the hardware. In the other side, if you need flexibility, scalability (and all other virtualization benefits :P) deploy a VM. It will be slower, but IMHO in some cases it's justified and the performance is not significantly slow.

Answer 6

You're trying to compare an operating system, software, and data installed on a certain physical hardware to that same operating system, software, and data installed by itself inside a hypervisor on the same original hardware. This comparison is just not valid, because almost no one does this. Of course that would likely be slower. Thankfully, it completely misses the most common point of why you virtualize servers at all.

A better example here is to look at two (or more!) older servers in your data center. Look for servers that are performing reasonably well, but are old now and coming up on their refresh cycle. Because these servers already perform well on older hardware, thanks to Moore's law anything new you get is gonna be way over-specced.

So what do you do? It's simple. Rather than buying two new servers you buy just one, and then migrate both of your old servers to the same physical new device. When preparing to purchase your new server, you plan so that you have enough capacity to not only handle the load from both older servers but also any load from the hypervisor (and maybe a little extra, so that you can still get a performance boost and can allow for growth).

Now let's stretch this a little further. Since these are old servers, perhaps you were looking at a couple simple $2000 pizza box servers to replace them. Chances are, even one of these pizza boxes could still easily handle both of your old servers. But now let's say you decide to spend $10000 or more on some real hardware instead. Now you have a device that can easily handle as many as a dozen of your existing servers (depending on how you handle storage), at the cost of only 5. And for this you have the added benefits of only managing one physical server, decoupling your software from your hardware (ie: hardware refresh is now less likely to need a new windows license), you save a ton on power, and your hypervisor can give you better information on performance than you've had in the past. Get two of these and depending on how big you are maybe your entire data center is down to just two machines, or perhaps you want to use the second server as a hot standby to tell a better high-availability story.

My point here is that it's not about performance at all. I would never take a perfectly good production server and virtualize it alone to equivalent hardware just because. It's more about consolidation and cost savings. That means you're moving your server to different hardware, and that in turn means you need to take the time to size that hardware appropriately, including accounting for the hypervisor penalty. Yes, you might need slightly more computing power in total than if each of those machines were on their own physical device (hint: you probably don't), but it's gonna be a whole lot cheaper and more energy efficient to run one physical server than it is to run two or more.

Answer 7

It seems Microsoft has done some benchmark testing using BizTalk server, and SQL Server in different configurations in this regard. See link below:

http://msdn.microsoft.com/en-us/library/cc768537(v=BTS.10).aspx

Answer 8

Sorry to disagree with TomTom.

I've been using VMware Workstation for a while mainly on Windows XP, Windows Vista and now Windows Seven native systems to run different Windows flavors as well as Ubuntu.

Yes, a virtualized environment is slower than a native system and that may be in a range of 5 up to 100 %.

The main problem isn't that much the CPU load but the physical memory lack.

Let's say you've a Windows Seven 64 Ultimate running on a 4 Gb system that when idle needs almost 1.5 Gb and uses ~ 10% of the CPU. Launching the extra layer of VMware will cost you ~ 300 Kb and the CPU loads will climb up to ~ 20%. Then launching a virtual system within VMware will request at a minimum the amount of memory you defined for that virtual machine that is a minimum of 1 Gb for any decent system. Then you'll see the CPU load ~ 60 % if the virtual machine is Ubuntu and ~ 80 % for any flavor of recent Windows OS.

Now, you'll start different apps within that virtual machine.

If the amount of memory you've set for that virtual machine is not enough, the virtualized system will start to swap, then dramatically slowing down its overall performance and responsiveness.

If the sum of the amount of memory you've set for that virtual machine plus the amount of memory needed for your native system is above the amount of memory of your native system, then it's your native system that is going to swap, slowing down both the native and virtualized system.

So, it first depends of the balance of the memory needed for both the native and the virtualized machines.

Now it's almost the same with the CPU load. If a virtualized app needs a huge CPU load, and a native app needs also a huge CPU load, your native system will have to manage the priority and balance the CPU charge between its different apps, the virtualized system being nothing but an app but that phenomenon is a classical CPU load problem that you can trick with app priorities.

So, my first advice if you need to use virtualization is to put a bunch of memory in your machine, whatever the OS you use natively or within a virtual machine.

Just my 2 cents.

Best regards.

Answer 9

In my experience virtual machines are always a lot slower than physical ones OUT OF THE BOX.

You will only notice it when running applications that hit the disk and tax the CPU a lot. I have run many databases and webservers on virtual machines and as an end user and the feedback from other endusers (ie: accessing the app from a remote web browser) there is quite a big lag when using virtual machines.

Of course a properly configured virtual machine may come to 80% (I don't know the real number) or whatever of the physical machine's speed, but you end up having to really dig deep into what the application is doing and how the virtual machine works. So I guess it is a cost equation of how valuable your time to configure VMs verses just buying and hosting a new server.

For me virtual machines are NOT ABOUT PERFORMANCE, but about being easier to manage and for hosting several low performance VMs of course.