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    <p>I would argue that all else being equal, and this code truly being CPU-bound and not benefitting from any memory sharing that running multiple processes on the same machine would provide, you should <strong>opt for the single core machines</strong> rather than multi-core machines. </p> <p>Reasons:</p> <p><strong>Isolate fault domains</strong></p> <p>Scaling out rather than up is better to do when possible because it naturally isolates faults. If one of your small nodes crashes, that only affects one process. If a large node crashes, multiple processes go down.</p> <p><strong>Load balancing</strong></p> <p>Windows Azure, like any multi-tenant system, is a shared resource. This means you will likely be competing for CPU cycles with other workloads. Having small VMs gives you a better chance of having them distributed across physical servers in the datacenter that have the best resource situation at the time the machines are provisioned (you would want to make sure to stop and deallocate the VMs before starting them again to allow the Azure fabric placement algorithms to select the best hosts). If you used large VMs, it would be less likely to find a suitable host with optimal contention to accommodate many virtual cores.</p> <p><strong>Virtual processor scheduling</strong></p> <p>It's not widely understood how scheduling a virtual CPU is different than scheduling a physical one, but it is something worth reading up on. The main thing to remember is that hypervisors like VMware ESXi and Hyper-V (which runs Azure) schedule multiple virtual cores together rather than separately. So if you have an 8-core VM, the physical host must have 8 physical cores free <em>simultaneously</em> before it can allow the virtual CPU to run. The more virtual cores, the more unlikely the host will have sufficient physical cores at any given time (even if 7 physical cores are free, the VM cannot run). This can result in a paradoxical effect of causing the VM to perform worse as more virtual CPU cores are added to it. <a href="http://www.perfdynamics.com/Classes/Materials/BradyVirtual.pdf">http://www.perfdynamics.com/Classes/Materials/BradyVirtual.pdf</a></p> <p>In short, a single vCPU machine is more likely to get a share of the physical processor than an 8 vCPU machine, all else equal.</p> <p>And I agree that the pricing is basically the same, except for a little more storage cost to store many small VMs versus fewer large ones. But storage in Azure is far less expensive than the compute, so likely doesn't tip any economic scale.</p> <p>Hope that helps.</p>
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    1. POAzure VM pricing - Is it better to have 80 single core machines or 10 8-core machines?
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    1. COIn our load-tests we observed this "paradoxical effect"
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