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