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context. In this way, CMT processors use what is wasted (stall) time on other
processors to continue doing useful work.
This feature is highly effective whether Logical Domains are in use or not.
Nonetheless, a recommendation for Logical Domains is to reduce cache misses
by allocating domains so they do not share per-core L1 caches. The simplest way
to do so is to allocate domains with a multiple of the CPU threads per core—for
example, in units of 8 threads on T2-based systems. This approach ensures that
all domains have CPUs allocated on a core boundary and not shared with another
domain. Actual savings depend on the system's workload, and may be of minor
consideration when consolidating old, slow servers with low utilization.
Logical Domains are implemented using a very small hypervisor that resides in
firmware and keeps track of the assignment of logical CPUs, RAM locations, and
I/O devices to each domain. It also provides logical channels for communication
between domains and between domains and the hypervisor.
The Logical Domains hypervisor is intentionally kept as small as possible for
simplicity and robustness. Many tasks traditionally performed within a hypervi-
sor kernel (such as the management interface and performing I/O for guests) are
offloaded to special-purpose domains, as described in the next section.
This scheme has several benefits. Notably, a small hypervisor is easier to de-
velop, manage, and deliver as part of a firmware solution embedded in the plat-
form, and its tight focus helps security and reliability. This design also adds re-
dundancy: Shifting functions from a monolithic hypervisor to privileged domains
insulates the system from a single point of failure. As a result, Logical Domains
have a level of resiliency that is not available in traditional hypervisors of the
VM/370, z/VM, or VMware ESX style. Also, this design makes it possible to lever-
age capabilities already available in Oracle Solaris, providing access to features
for reliability, performance, scale, diagnostics, development tools, and a large API
set. It has proven to be an extremely effective alternative to developing all these
features from scratch.
Domains are used for different roles, and may be used for Logical Domain in-
frastructure or applications. The
control domain
is an administrative control
point that runs Solaris or OpenSolaris and the Logical Domain Manager services.
It has a privileged interface to the hypervisor, and can create, configure, start,
stop, and destroy other domains.
Service domains
provide virtualized disk and
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