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

[lifeboy]

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:


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]

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]

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]

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]

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.

 

[arxaios]

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 ...!!

 

[fiona]

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]

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..!!!

 

[Dunuin]

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]

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]

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/

 

[lifeboy]

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/

Yes, thanks, will do

 

[Neobin]

The top should be 100% and the bottom 0%.

Feel free to open an enhancement request on our bug tracker: https://bugzilla.proxmox.com/

Already existing:
https://bugzilla.proxmox.com/show_bug.cgi?id=4254

 

[LnxBil]

We are very used to other tools showing the actual CPU usage, so I'm surprised that this is a percentage graph.

What tools? What display type? I've never seen anything else than percentage. The term "usage" implies percentage for CPUs.

 

[lifeboy]

What tools? What display type? I've never seen anything else than percentage. The term "usage" implies percentage for CPUs.

40 CPU's, number 40 in the graph. Not percentages.

Go to a VM however:


6 vCPU's assigned, the graphs shows 7. Why on earth would anyone think that this is a percentage graph?

 

[leesteken]

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).

 

[lifeboy]

It's percentages both. CPU usage in both graphs is always percentage (between 0 and 100 inclusive).

Then it must be changed to say percentage use. There is an enhancement request open for this.

 

[LnxBil]

View attachment 42904

40 CPU's, nubmer 40 in the graph. Not percentages.

Go to a VM however:

View attachment 42905
6 vCPU's assigned, the graphs shows 7. Why on earth would anyone think that this is a percentage graph?

You see that in every textual representation of the data in your screenshot, the term % is ALWAYS present when the term usage is also here (and it is available).

 

[fiona]

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

 

[lifeboy]

What tools? What display type? I've never seen anything else than percentage. The term "usage" implies percentage for CPUs.

I know the matter has been resolved, but just for reference, here's what I was referring to:

~# uptime
14:17:19 up 21 days, 19:44, 1 user, load average: 5.37, 5.36, 5.55

This refers to CPU's, not percentages.