ESXi: Could not power on VM: Admission Check Failed

After building a dev server on the newly free ESXi software from VMware I tried to add a couple VM’s – The first virtual machine ran great however the second would fail with the following error:

“Could not power on VM: Admission check failed for memory resource”

After some digging I found the following thread on VMWare forums:

It seems in my case my server which only has 1GB of Ram currently could not support the overhead of running 2 virtual machines due to a system memory reservation.  The fix was easy, buy more Ram.  However, I wanted to have both VM’s running right away and further down the page I found a fix:

The Memory Reservation on the Resource Allocation tab reflects the portion of memory that ESXi thinks it can give to VMs (i.e. Virtual Machines memory – VM memory overhead). With ESX regulary that number is around 200 MB but with ESXi its up around 700 MB. Not such an issue when you have 16 GB, but it is with 1 / 2 GB. To free up some more memory

1) Go to Configuration \ system resource allocation
2) Click on advanced.
3) Find the VIM object and change the reservation from 512 MB to 192 MB.

Sure enough I could now launch two virtual machines however I was then greeted by a second error:

“Could not power on VM: Admission check failed for cpu resource”

Here again my server only had 1 CPU with Hyperthreading (an older xeon).

Some experimenting resulted in a solutiion:

On each of my virtual machines I went into edit settings – then into the Resources Tab.

I configured my CPU with a reservation of 0Mhz, then I verified that Advanced CPU had Hyperthreaded Cored Shared Mode set to Any.  I was then able to launch both of my virtual machines without issue.