Multi-threading in a VMWare virtual machine environment

Question

We scale our single-threaded app by running it in separate vm's - each instance is configured to work on a particular partition of the overall workload. An idea has been circulating that we can get better performance by adding threads to some parts of the app, though we would not be eliminating the current vm dependence.

Is architecting threading for an app that has been designed for a vm environment different than for an app designed for a non-vm environment? My primary concern is that for every thread designed into the app the actual number of threads that may be spun up per machine is a function of the number of vm instances running on the machine and may actually lead to performance degradation.

Thanks in advance.

Edit: By vm above I mean a virtual machine as provided by VMWare.

Answer 1

I think your concerns about "performance degradation" are warranted. If you are running multiple VMs on a machine and add multiple threads to the VMs, you are most likely going to be increasing the context switching only -- not getting more work out of a VM.

It depends a lot on the jobs you are running of course. If they are IO bound, then adding threads may give you better parallelization. However, if they are CPU/computation bound, then you will most likely not get a win and most likely see a drop in performance.

Answer 2

Is architecting threading for an app that has been designed for a vm environment different than for an app designed for a non-vm environment?

Not IME, but then I don't tend to write CPU-intensive apps - I most often thread off to get stuff out of the GUI and to simplify design for multiple users/clients. I just design the apps as if I am on a native OS.

I don't know how the threads are mapped. I have an XP VM running now. The XP task manager shows 518 threads, the host, (Vista 64), task manager shows only 11 threads for 'VMware Workstation VMX', though there are some 22 other threads for NAT Sevice, VMnet DHCP, Tray Process etc. I have 2 'processors' assigned to the VM to give any multithreading bugs more chance of showing up.