48 lines
1.7 KiB
ReStructuredText
48 lines
1.7 KiB
ReStructuredText
|
==============
|
||
|
|
Overcommitting
|
||
|
|
==============
|
||
|
|
|
||
|
|
OpenStack allows you to overcommit CPU and RAM on compute nodes. This
|
||
|
|
allows you to increase the number of instances you can have running on
|
||
|
|
your cloud, at the cost of reducing the performance of the instances.
|
||
|
|
OpenStack Compute uses the following ratios by default:
|
||
|
|
|
||
|
|
* CPU allocation ratio: 16:1
|
||
|
|
* RAM allocation ratio: 1.5:1
|
||
|
|
|
||
|
|
The default CPU allocation ratio of 16:1 means that the scheduler
|
||
|
|
allocates up to 16 virtual cores per physical core. For example, if a
|
||
|
|
physical node has 12 cores, the scheduler sees 192 available virtual
|
||
|
|
cores. With typical flavor definitions of 4 virtual cores per instance,
|
||
|
|
this ratio would provide 48 instances on a physical node.
|
||
|
|
|
||
|
|
The formula for the number of virtual instances on a compute node is
|
||
|
|
``(OR*PC)/VC``, where:
|
||
|
|
|
||
|
|
OR
|
||
|
|
CPU overcommit ratio (virtual cores per physical core)
|
||
|
|
|
||
|
|
PC
|
||
|
|
Number of physical cores
|
||
|
|
|
||
|
|
VC
|
||
|
|
Number of virtual cores per instance
|
||
|
|
|
||
|
|
Similarly, the default RAM allocation ratio of 1.5:1 means that the
|
||
|
|
scheduler allocates instances to a physical node as long as the total
|
||
|
|
amount of RAM associated with the instances is less than 1.5 times the
|
||
|
|
amount of RAM available on the physical node.
|
||
|
|
|
||
|
|
For example, if a physical node has 48 GB of RAM, the scheduler
|
||
|
|
allocates instances to that node until the sum of the RAM associated
|
||
|
|
with the instances reaches 72 GB (such as nine instances, in the case
|
||
|
|
where each instance has 8 GB of RAM).
|
||
|
|
|
||
|
|
.. note::
|
||
|
|
Regardless of the overcommit ratio, an instance can not be placed
|
||
|
|
on any physical node with fewer raw (pre-overcommit) resources than
|
||
|
|
the instance flavor requires.
|
||
|
|
|
||
|
|
You must select the appropriate CPU and RAM allocation ratio for your
|
||
|
|
particular use case.
|