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Question about overcommit of cpu and ram

asked 2016-12-19 05:56:53 -0500

marten gravatar image

I have a question about nova now, as we all know that the physical cpu and ram can be assigned to vCPU and vRam exceed its actual capacity, my question is that whether the compute node will crashed or not when all the vms in the node are runing in a full load way assumed that the physical node resources have been completed assigned to the vms ? If there exist any mechanism to avoid this happen ? Thank you in advanced!

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answered 2016-12-19 10:20:35 -0500

sgordon gravatar image

At first the system will attempt to use swap if available before eventually killing processes due to Out of Memory (OOM), so the compute node itself will not crash if it reaches a full memory load but individual processes including the qemu-kvm processes that represent the guests can and will.

Techniques like memory ballooning can help alleviate such issues but they are not currently utilized by OpenStack Compute (Nova). Kernel Samepage Merging (KSM) is another technique that can help, albeit at the expense of some CPU usage, and can be used with Nova. Refer to for more information about KSM.

Neither of these techniques can completely eliminate the OOM killer if you overcommit too heavily, it is important to set the overcommit ratios after careful consideration of the real characteristics of your workload and environment.

You can find some more information about hypervisor tuning regarding memory at

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Thank you very much and the answer solve my confusion perfectly! Now I want to now more about whether the nova will do anything to rescue the qeum-kvm automatically when it was killed ?

marten gravatar imagemarten ( 2016-12-19 19:35:02 -0500 )edit

If the nova doesn't exist any mechanism to balance load once the vms have been launched?

marten gravatar imagemarten ( 2016-12-19 19:37:40 -0500 )edit

These are both considered out of scope for Nova at this time, though there are other OpenStack projects which attempt to provide some such facilities by exercising the Nova APIs.

sgordon gravatar imagesgordon ( 2016-12-28 12:17:58 -0500 )edit

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Asked: 2016-12-19 05:55:16 -0500

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Last updated: Dec 19 '16