Memory Management IT managers must use care to create VMotion clusters composed of Nehalem-based processors and anticipate the release of ESX 4.0 to take full advantage of the processor's new memory management features."A general guideline is to provide 4GB of RAM per hardware thread per logical processor," said VMware's Brunner. A four-socket system with quad-core Nehalem processors could be easily accommodated in the new memory address scheme. Aside from supporting dense VM deployments with large RAM support, Nehalem processors use another significant update dubbed VT-d (Virtualization Technology for Directed I/O) that connects dedicated DMA (direct memory access)-capable I/O resources for virtual machines. Indeed, one of the big challenges of virtualization has been handling I/O. VT-d allows the direct assignment of a VM to a physical device. As implemented, VT-d seeks to reduce the performance overhead incurred as the hypervisor moderates state among the guest VMs. For example, each time a network interface on the physical server processes a packet, it goes through an interrupt process in the operating system. In virtualization, the problem is that every interrupt requires that a VM exit to the VM manager and back again. Using VT-d, the VM is directly assigned to the networking device, thereby bypassing the interrupt and exit process and significantly cutting network I/O overhead.
And these features are prodigious. The Nehalem-based processors can now handle up to 18 8GB DIMMs, for a total of 144GB of RAM per physical host.