By far the most common task assignment policies used in the immediate
dispatching model are `Random` and `Round-Robin`. The `Random` policy assigns an incoming job to each server with probability
, where is the number of servers, while the `Round-Robin`
policy assigns jobs to servers in a cyclic order. `Random` and
`Round-Robin` are simple, but they neither maximize utilization of
the hosts, nor minimize mean response time.

In the immediate dispatching model, the `Shortest-Queue` policy
has been shown to be optimal under various constraints and objective
functions under the conditions that the service demand has an exponential
distribution and job sizes are not known a priori
[49,106,124,192,209].
Under the `Shortest-Queue` policy, incoming jobs are immediately
dispatched to the host with the fewest number of jobs.

In the central queue model, the `M/G/k/FCFS` policy has been
proven to minimize mean response time and maximize utilization under
the condition that the service demand has an exponential distribution
and job sizes are not known a priori [210]. The
`M/G/k/FCFS` policy holds all jobs at the central queue, and when
a host becomes free, it receives a job from the central queue in the
order of their arrivals (first come first served, FCFS). The `M/G/k/FCFS` policy is provably identical to the `Least-Work-Remaining` policy in the immediate dispatching model [64]. The
`Least-Work-Remaining` policy assumes that the jobs sizes are known a priori,
and it sends each job to the host with the least total remaining work.

While policies like `Shortest-Queue` and `M/G/k/FCFS` perform
well when job sizes have an *exponential* distribution (or
distribution with increasing failure rate), they perform *poorly*
when the job size distribution has higher variability
[106,201]. In such cases, it has been shown
analytically and empirically that the `Dedicated` policy far
outperforms these other policies with respect to minimizing mean
response time [65,174]. In the `Dedicated` policy, some hosts
are designated as the ``short hosts'' and others as the ``long
hosts.'' Short jobs are always sent to the short hosts and long jobs
to the long hosts. Observe that the `Dedicated` policy is defined
for both the immediate dispatching model and the central queue model,
and the two models are identical under the `Dedicated` policy.
The `Dedicated` policy is popular in practice (e.g. Cornell Theory
Center) where different host machines have different duration
limitations: 0-1/2 hour, 1/2 - 2 hours, 2 - 4 hours, etc., and
users must specify an estimated required service demand for each job.
The `Dedicated` policy performs well when job sizes have high variability
because it isolates shorts
jobs from the long jobs, as waiting behind the long jobs is costly.
The `Dedicated` policy is also popular in supermarkets and banks,
where an ``express'' queue is created for ``short'' jobs.

The `Dedicated` policy requires to know job sizes upon their
arrivals; however, even when the job size is not known, a policy
similar to `Dedicated`, known as the `TAGS` policy (task
assignment by guessing size) works almost as well when job sizes have
high variability. Like `Dedicated`, the `TAGS` policy
significantly outperforms other policies that do not segregate jobs by
size [64].