Solving Very Slow Visual Studio Build Times in VMWare

ClockIn 2009, we chose 15″ MacBook Pro BTO over Dell, HP and Lenovo offerings. Apple offered us the best hardware and equivalent lease terms but with much simpler servicing done by ETF rather than (often incorrect) paper statements and checks. 99% of the time we ran this MacBooks with Windows 7 under Boot Camp. When I started doing some iOS development, I ran Snow Leopard and Xcode under VirtualBox and when I got fed up with the flakiness, I used VMware Workstation. OS X didn’t run very well under virtualization mostly because accelerated Quartz Extreme drivers don’t exist for VMware Workstation. Still, it actually worked well enough and was much more convenient than dual booting — which is such a huge time suck. When the lease period ended, we renewed with Apple and decided to just use OS X as the host environment for a variety of reasons:

The transition went very smoothly. I was using a VM with Windows Server 2008 and Visual Studio 2010 for primary .NET web development. Configuring IIS Express to serve outside of localhost bound to a host-only adapter is great for cross-browser testing, but it can be even more useful to enable remote access to Fiddler and point external browsers to the Fiddler proxy running in the VM to get both client debugging and HTTP sniffing at the same time. All of this was working out great.
 
At the start of October I downloaded Windows Server 2012 and Visual Studio 2012 and created a new development VM. In order to minimize disk storage, I moved my source tree to a folder in the host OS X environment and exposed it via the “Shared Folders” feature of VMWare Fusion 5.x to Windows. At the same time I started working on a new project.

 

The Slowening

Once the project grew to 20, 30 and 50k lines of C# code, the build times started to become horrifically slow. When combined with running unit tests, build became a big time for a bathroom break or cup of coffee event like building a project in C 15 years ago. Builds would show cdc.exe running at ~50% CPU (e.g. 1 core) and some other stuff totaling ~70% CPU. The VM was not memory bound and network IO was minimal. This was my first substantial project using Code First EF, so I thought maybe the complex object graph is just hard for the C# compiler to deal with.
 
After a few weeks of increasingly painful build times, I was looking at breaking my solution up so that I could build against pre-compiled DLLs — anything to make it go faster. I ran across a post on SuperUser:

… (Full disclosure: I work on VMware Fusion.)

I have heard that storing the code on a “network” drive (either an HGFS share or an NFS/CIFS share on the host, accessed via a virtual ethernet device) is a bad idea. Apparently the build performance is pretty bad in this configuration.

Oh really? Hmmm. Maybe it isn’t that my class libraries are so complex but something else is going on. Here are some empirical measurements of rebuild time of an actual solution:

VMWare shared folder: 	50 sec
OS X SMB share: 	18 sec
within virtual disk:	 9 sec

Wow. Problem solved. Incremental builds are basically instantaneous and a full rebuild takes 9 seconds when the code is hosted inside the VM image. Not only does hosting the source code within the VM virtual disk make the build go 5.5x faster, the CPU time of csc.exe goes way down. I don’t know how the VMWare shared folder is implemented. It appears as a mapped drive to a UNC name to Windows but it is very slow. Moral of the story is just don’t host your source code on the host machine with VMWare. The performance penalty is just not worth it. If you need to share the source code tree inside the VM with the host OS, create a file share from the VM to the host over a host-only adapter.

 

Update

I can confirm this is still a problem in VMWare Fusion 6. I’m hoping maybe the new SMB implementation in Mavericks might greatly improve the performance of sharing source code from the host OS to the VM.

Update

I just updated my virtual machine to Windows Server 2012 R2 (aka Windows 8.1 server). It is running on VMWare Fusion 6. The build time of this large project is now 15 seconds over VMWare Shared Folders. Significant remaining issue is that Visual Studio uses a whole core of CPU to do nothing — just having a large solution open, not editing anything.

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