Why is Visual Studio so slow when using 2 CPU cores?

[stkris]

Hi!

My laptop runs Debian and I use VirtualBox to run Windows XP and Visual Studio. But using 2 CPUs just make Visual Studio under XP really slow.

Stepping trought a particular function in the debugger I use 1:47 minutes with 1 CPU enabled, and 10:03 minutes with 2 CPUs enabled in VirtualBox.

On the contrary - a very CPU intensive application works twice as fast as you would expect when enabling 2 CPUs.

Can anybody think of any reason why VS 2008 is so slow on my setup? The log files show no exeptions or other error messages.

My system:
The Laptop:
* Intel(R) Core(TM)2 Duo CPU T8300 @ 2.40GHz
* Linux a 2.6.31-1-amd64 #1 SMP Mon Nov 16 04:44:38 UTC 2009 x86_64 GNU/Linux

The VirtualBox:
* VirtualBox version Installed: 3.1.2-56127_Debian_lenny
* Windows XP SP3 with this kernel: ACPI Multiprocessor PV v. 5.1.2600

[togakangaroo]

I'm getting the same problem running Windows 7 as guest and host.

I gave 4 gig ram and 4 cores to the VM and it slowed to a crawl! Much better with only one core.

[stkris]

So it looks to be independent of the host OS or client windows version.

[togakangaroo]

At least as far as Windows is concerned, yes it seems to be independent

[Martin]

You shouldn't assign all your CPU cores to a VM with a high I/O load. As soon as you need to access the network or disk, the host OS needs at least one CPU core to move the data, therefore the guest needs to pause to release the CPU.
When the guest has mostly high CPU load, you don't need to get "outside" the VM, so the guest has much more CPU resources available.

[stkris]

I did a quick test on my Quad Core box running Debian (lenny):

Code: Select all

virtual@ulet:~$ cat /proc/cpuinfo 
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 15
model name	: Intel(R) Core(TM)2 Quad CPU    Q6600  @ 2.40GHz

virtual@ulet:~$ uname -a
Linux ulet 2.6.26-2-686-bigmem #1 SMP Tue Mar 9 18:01:52 UTC 2010 i686 GNU/Linux

Installed: 3.1.4-57640_Debian_lenny

I ran a "rebuild all" in Visual Studio 2008 three times for each CPU count.
And then I tried three more so that VS2008 could use 1 or 3 threads while building the project.

1 CPUs 1 threads 1: 12.33
1 CPUs 1 threads 2: 12.44
1 CPUs 1 threads 3: 12.00

1 CPUs 3 threads 1: 12.01
1 CPUs 3 threads 2: 10.78
1 CPUs 3 threads 3: 11.81

2 CPUs 1 threads 1: 14.20
2 CPUs 1 threads 2: 13.37
2 CPUs 1 threads 3: 13.66

2 CPUS 3 threads 1: 12.98
2 CPUS 3 threads 2: 11.56
2 CPUS 3 threads 3: 10.79

3 CPUs 1 threads 1: 17.84
3 CPUs 1 threads 2: 15.51
3 CPUs 1 threads 3: 16.34

3 CPUs 3 threads 1: 15.38
3 CPUs 3 threads 2: 13.55
3 CPUs 3 threads 3: 12.80

4 CPUs 1 threads 1: 28.90
4 CPUs 1 threads 2: 24.62
4 CPUs 1 threads 3: 23.39

4 CPUs 3 threads 1: 27.62
4 CPUs 3 threads 2: 22.06
4 CPUs 3 threads 3: 25.67

As you can see the build time increases with each CPU I make available to VirtualBox.

Increasing the numbers of threads VS uses helps a bit, but does not stop the decline.

The best build time was with 1 CPU and 3 threads, and the worst with 4 CPUs and 1 thread.

I belive I will set all my VMs to use 1 CPU for now.

[Samuel13]

see http://forums.virtualbox.org/viewtopic.php?f=2&t=29899