1 CPU = 5 vCPUs is okay?

Not_Oles · October 2020

From: https://nb.fedorapeople.org/cvsfedora/web/html/docs/virtualization-guide/f12/en-US/html/sect-Virtualization_Guide-Tips_and_tricks-Overcommitting_with_KVM.html

Virtualized CPUs are overcommitted best when each virtualized guest only has a single VCPU. The Linux scheduler is very efficient with this type of load. KVM should safely support guests with loads under 100% at a ratio of 5 VCPUs Overcommitting single VCPU virtualized guests is not an issue.

So, 1 CPU = 5 vCPUs is okay?

deank · October 2020

Doesn't matter how "efficient" it is.

One PMSing benchmark junkie is all you need to break down a node.

Not_Oles · October 2020

Also

Assigning guests VCPUs up to the number of physical cores is appropriate and works as expected.

Not_Oles · October 2020

@deank said: PMSing benchmark junkie

We don't have anybody like that here!

PMSing = yes
benchmark junkie = yes
PMSing + benchmark junkie = no

Not_Oles · October 2020

DOI: 10.1109/CLOUD.2012.131

Not_Oles · October 2020

On page 29 of the cited article:

Proposed approach using a running example. For the 
sample of group 32 VMs presented in this analysis, total
number of vCPUs requested is 99. We assume that these
VMs are to be provisioned on a PM with 16 physical CPU
cores that can support a maximum of 32 vCPUs. **Thus, the
vCPU capacity is over-committed by a factor of 99/32 or
3.09.** For this group of VMs, we predict the probability
of exceeding a set threshold utilization and estimate the
risk over a 24 hr time window. Specifically, the analysis is
presented for two values of threshold utilization: 70% and
95%. Since the collected data contains utilization samples
at every 15 minutes interval, over a 24 hr window, total 96
samples are recorded. Figure 4 shows the **aggregate CPU
utilization** in each time interval and the set thresholds at
70% and 95% utilization. Observe, for the given group
of VMs, 70% threshold is exceeded by a small samples
although **none exceeds the 95% threshold.**

So, assuming similar use to the private cloud example from the cited article, it looks like 1 CPU = 3 vCPUs might be okay?

Here is maybe a more convenient citation to the article from which the above quote is taken: Biting Off Safely More Than You Can Chew: Predictive Analytics for Resource Over-Commit in IaaS Cloud.

_MS_ · October 2020

@Not_Oles said:
So, assuming similar use to the private cloud example from the cited article, it looks like 1 CPU = 3 vCPUs might be okay?

From the example, 16 pCPU cores = 32 provider vCPUs = 99 client vCPUs.
Which makes, 1 pCPU core = 2 provider vCPUs = 6 client vCPUs.

gks · October 2020

Interesting, how about RAM ratio?

Neoon · October 2020

Well, on NanoKVM/microLXC always gave a VM 1 core, so It wont trip any monitoring or performance issues if someone compiles stuff or uses his core. So a single user cannot impact the node's performance for affecting someone else.

Which is quite nice, so everything works flawless.
Of course you can over commit but you may end up in issues.

Not_Oles · October 2020

MS said:

@Not_Oles said:
So, assuming similar use to the private cloud example from the cited article, it looks like 1 CPU = 3 vCPUs might be okay?

From the example, 16 pCPU cores = 32 provider vCPUs = 99 client vCPUs.
Which makes, 1 pCPU core = 2 provider vCPUs = 6 client vCPUs.

It's my poor vocabulary. Sometimes I use the term "CPU" to mean "thread." I should be more clear. Thanks for highlighting my ambiguity. That's very helpful!

jarland · October 2020

@Not_Oles said: So, 1 CPU = 5 vCPUs is okay?

Sounds like pretty safe territory to me under average conditions.

Not_Oles · October 2020

@gks said:
Interesting, how about RAM ratio?

Section VI of the cited paper mentions related research involving memory and says that the analytical approach in the cited paper can be extended to run-time memory allocation.

Swap also needs to be considered.

jarland · October 2020

It's really interesting to see their recommendations on overcommitting memory. Truth be told the professional crowd are a lot less shy about overcommitting than the hobbyist crowd.

Not_Oles · October 2020

@Neoon said: NanoKVM/microLXC always gave a VM 1 core

Just to be certain, please let me ask: do you mean "1 core" in the sense of "1 thread" which would be 1 "processing unit" in the output of GNU nproc?

@Neoon said: Which is quite nice, so everything works flawless.

There is a guy who uses a desktop environment with Firefox and VScodium under LXC on one of my Xeon D 1521 servers. For awhile I kept asking him if everything works okay. Eventually he said, "I insist it works okay." So I stopped asking.

LXC doesn't get nearly the love that it deserves!

1 CPU = 5 vCPUs is okay?

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