1. A computer system, comprising: a cluster of computing nodes, each computing node comprising: a memory; a network interface device, which connects the computing node to a network interconnecting the computing nodes in the cluster; and at least one CPU, which is configured to run a respective virtual machine implementer program on the computing node, while communicating via the network interface device with other computing nodes in the cluster, wherein the other computing nodes run respective virtual machine implementer programs, so that the respective virtual machine implementer programs jointly support a shared virtual machine implemented concurrently on two or more of the computing nodes, wherein a guest operating system runs on the shared virtual machine, and instructions of the guest operating system are distributed for execution among the nodes as though the system were a single symmetric multiprocessing machine with shared memory.