[SOLVED] LXC with more cores assigned uses dramatically less CPU. Why?

lifeboy

Active Member
I have an interesting situation. An LXC running Power-mail-in-a-box has 4 cores assigned (with 8GB RAM and 100GB NVMe ceph pool storage).

The graph below shows the following:

1667290320364.png
The section from 9:32 to around 10:02 is when I only had 4 cores assigned. Before and after that time I had 12 cores assigned. When I assign the 12 cores, the CPU usage drops to practically always below 4 cores, but when I limit the cores to 4 only, the usage spikes to 12 and above. As can be seen, the general cpu usage it twice of more with 4 cores than with 12. What is happening here?
 

arxaios

New Member
Oct 31, 2022
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I do not know ....but that is wrong.....very wrong.! This shows that the Virtualization system is doing something wrong. And this exactly states what I have encountered on my previous post with the sudden freezing ...!! Proxmox is freezing at worse times when it is under stretch I suppose , which is very wrong for a virtualization system.It cannot handle these situations correctly...!! But anyway DO NOT USE limit values and leave it to Unlimited which I do not comprehend . Just try and please do post back..!!
 

fiona

Proxmox Staff Member
Staff member
Aug 1, 2019
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Hi,
I have an interesting situation. An LXC running Power-mail-in-a-box has 4 cores assigned (with 8GB RAM and 100GB NVMe ceph pool storage).

The graph below shows the following:

View attachment 42813
The section from 9:32 to around 10:02 is when I only had 4 cores assigned. Before and after that time I had 12 cores assigned. When I assign the 12 cores, the CPU usage drops to practically always below 4 cores, but when I limit the cores to 4 only, the usage spikes to 12 and above. As can be seen, the general cpu usage it twice of more with 4 cores than with 12. What is happening here?
the usage is in percent. In absolute values, 4% of 12 cores is the same as 12% of 4 cores ;)
 

arxaios

New Member
Oct 31, 2022
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1
the usage is in percent. In absolute values, 4% of 12 cores is the same as 12% of 4 cores ;)
No .....you are so wrong at this....obviously you do not know maths so if on 12cores is 4% then on 4cores it should be 1.33% equally..!!
 

fiona

Proxmox Staff Member
Staff member
Aug 1, 2019
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And if you use 0 cores, then the container suddenly has no workload at all ;)

Sorry, could not resist. To clarify: it's the percentage of assigned cores to the container. If you reduce the number of cores, the same workload needs to use more of the fewer available cores.
 
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arxaios

New Member
Oct 31, 2022
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You cannot use 0 cores ..!! What I want to say is that the system calculation or the workload given to the cores is not good THAT'S why a lot of people have freezes and glitches with it..!!! It has to be carefully designed because there is a debian OS underneath...!!
Obviously you haven't read what I wrote....please read it again carefully ...!!
 
Last edited:

fiona

Proxmox Staff Member
Staff member
Aug 1, 2019
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You cannot use 0 cores ..!! What I want to say is that the system calculation or the workload given to the cores is not good THAT'S why a lot of people have freezes and glitches with it..!!! It has to be carefully designed because there is a debian OS underneath...!!
Obviously you haven't read what I wrote....please read it again carefully ...!!
The usage calculation displayed in the UI is informational only. Freezes and glitches are completely independent of that and likely caused by other issues in the software/hardware stack. If you have a concrete issue, create a new thread with details about the issue and your setup. This thread is not about freezes.
 

arxaios

New Member
Oct 31, 2022
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My Friend it is not Independent at all...but as you say is for another thread...!! And I have just mention it ...!! One more thing...as you may ...Then the information is wrong...and optional and we shouldn't pay attention to it..!!!
 
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Dunuin

Famous Member
Jun 30, 2020
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I don't see what you don't get.

Let us say your LXC runs 2 processes that need 100% CPU time.

Assign that LXC 4 vCPUs and the CPU utilization is 50%, as 2 of 4 vCPUs are 100% utilized and 2 vCPUs are just 0% utilized.
Assign that LXC 12 vCPUs and it's just 16.6% CPU utilization, as 2 of 12 vCPUs are 100% utilized and 10 are 0% utilized.

So the more vCPUs you assign that LXC, the lower your LXCs total CPU utilizaton will be, as the workload stays the same, but the LXC got more resources to work with.
 

lifeboy

Active Member
Hi,

the usage is in percent. In absolute values, 4% of 12 cores is the same as 12% of 4 cores ;)
Ouch! We are very used to other tools showing the actual CPU usage, so I'm surprised that this is a percentage graph. The left hand side scale does not indicate that and it wrong then. The top should be 100% and the bottom 0%. As it is, the scale shows 14 at the top. Is that 14%?

I think this needs to be changed, as it's just wrong to show it like this. One can't even use it for comparison with other nodes.
 

fiona

Proxmox Staff Member
Staff member
Aug 1, 2019
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Ouch! We are very used to other tools showing the actual CPU usage, so I'm surprised that this is a percentage graph. The left hand side scale does not indicate that and it wrong then. The top should be 100% and the bottom 0%. As it is, the scale shows 14 at the top. Is that 14%?

I think this needs to be changed, as it's just wrong to show it like this. One can't even use it for comparison with other nodes.
Yes, it is 14% (of the assigned number of CPUs). Feel free to open an enhancement request on our bug tracker: https://bugzilla.proxmox.com/
 

leesteken

Famous Member
May 31, 2020
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40 CPU's, nubmer 40 in the graph. Not percentages.
6 vCPU's assigned, the graphs shows 7. Why on earth would anyone think that this is a percentage graph?
It's percentages both. CPU usage in both graphs is always percentage (between 0 and 100 inclusive).
 
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fiona

Proxmox Staff Member
Staff member
Aug 1, 2019
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Then it must be changed to say percentage use. There is an enhancement request open for this.
A patch to use show that it's percentage has been applied in git and will be in the next pve-manager version. But that is not what the enhancement request is about ;)
 

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