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Parallels Containers -> Virtuozzo
Typically users also care for quality of service of their software, i.e. how well their services are working on utilized node and how fast they respond to external requests. The metrics of quality of service are average/min/max response time, 99.9% requests response time and similar. When a hardware node is capable to handle the load, these metrics either do not grow much with bigger number of containers, or grow linearly. When a hardware node becomes over utilized, these metrics typically start to degrade exponentially (due to memory swap out).
Below we summarize differences between containers (OpenVZ, Parallels ContainersVirtuozzo, Solaris Zones) and hypervisor (ESX, Xen, KVM, HyperV) which affect density and make containers to be more suitable for high density environments then virtual machines.
== What makes containers to be perfectly suitable for high density? ==
* Containers CPU scheduler is interactive system-wide. If some process is woken up by an external event (like networking request) CPU scheduler can quickly preempt current CPU hog task and schedule interactive one. This is not true with VMs where hypervisor doesn't know the nature of tasks inside the VM and whether VM should preempt another VM on even or not. In high-density environments it is particularly important to make sure that interactive tasks get CPU fast enough and provide low latency response to users.
* Parallels Virtuozzo Containers product goes a step further by introducing container templates, used as a basis for all containers, and a special copy-on-write filesystem, which makes sure that original template is kept untouched and container gets its own private copy of template file when it tries to modify it. As a result all common files are shared across the containers and present on disk and in memory caches in a single instance. This saves memory, reduces I/O and makes L2/L3 caches work more effective.
== Why Virtual Machines are not that good? ==
