Relaxed Co-scheduling a clarification – Danish myth busted.

I know this is old news, but at a recent meeting at vmware 4 different customers agreed that they were taught something different by instructors in Denmark about Co-scheduling.

The myth that all 4 of us believed in states that:

“An 8 vCpu machine can only get cpu time, if 8 cores are available to the scheduler”, so if 7 cores were available the machine would wait, which you could see from the cpu-ready counters.

However since ESX 3.X vmware has done something called “relaxed co-scheduling”, meaning that if a machine has 8 vCpu’s but only has a single threaded application it can run if fewer than 8 cores were available to scheduler.

See Duncans post about this from 2008!

We’ve done a load test of a SharePoint 2013 farm which consisted of 2 front-end servers each with 24 vCpu’s that were run on a 4-way server with 8 cores in each processor, on vSphere 5.0.

loadtest showing cpu ready

Had strict co-scheduling still existed we should have seen CPUREADY %’s above 50% instead we got a peak at 33%. That however shows that we’re still over comitting the host too much.