New app install CPU share : change default

[robi]

They're just upper limits of overall CPU(s) available for the app container. Just like RAM.

[humptydumpty]

@robi okay, but here are some more questions/concerns.

• what happens when an app reaches the limit (say 20%)? Does it throttle speeds or crash like when an app runs OOM?

• we have “recommended/minimum” RAM figures to use for each app, but no info on CPU usage. If the staff assign a global default, then some apps might need more cpu shares. In this case, how do I figure out what’s the lowest number of shares we should set to run the app reliably?

• how would this work if it’s percentage based since each VPS has different resources? 10% might work fine on one VPS but crash on another ($5 droplet).

[robi]

@humptydumpty said in New app install CPU share : change default:

@robi okay, but here are some more questions/concerns.

• what happens when an app reaches the limit (say 20%)? Does it throttle speeds or crash like when an app runs OOM?

No, it just slows down.

• we have “recommended/minimum” RAM figures to use for each app, but no info on CPU usage. If the staff assign a global default, then some apps might need more cpu shares. In this case, how do I figure out what’s the lowest number of shares we should set to run the app reliably?

There is no need to that. See above.

• how would this work if it’s percentage based since each VPS has different resources? 10% might work fine on one VPS but crash on another ($5 droplet).

Again, it cannot crash from CPU limits. Only RAM and how the app handles running out of memory.

CPUs are shared in time, up to % of overall CPU(s).

RAM is not shared, but used in chunks up to an individual and then overall physical limit.

[humptydumpty]

@robi thanks for the explanation!

[robi]

@humptydumpty You're welcome.

Don't be afraid to test it by setting the CPU shares really low and observe what happens.

[girish]

This doesn't actually allocate CPU the way you think. As @robi pointed out, it allocates cpu shares - how much to allocate to which app when resources are short. By default, app sees all the CPU

I will refer https://docs.docker.com/config/containers/resource_constraints/#cpu :

Set this flag to a value greater or less than the default of 1024 to increase or reduce the container's weight,
and give it access to a greater or lesser proportion of the host machine's CPU cycles. **This is only enforced
when CPU cycles are constrained.** When plenty of CPU cycles are available, all containers use as much CPU
as they need. In that way, this is a soft limit. --cpu-shares doesn't prevent containers from being scheduled in
Swarm mode. It prioritizes container CPU resources for the available CPU cycles. It doesn't guarantee or 
reserve any specific CPU access.

In practice, I have found this very hard to "measure". I tried to summarize the above in https://docs.cloudron.io/apps/#cpu-shares (screenshot) but maybe it's not clear enough.

[timconsidine]

No, I think I understand the concept of CPU shares.
I just think it is bizarre / bad practice to set the default to 50%.

If the setting has a practical impact when overall system is under load, then why set it high for all apps as a default.
If it doesn't have a practical impact, why bother having it OR why set it at 50% ?

Anyway, it doesn't really matter, I guess.

[jdaviescoates]

@timconsidine said in New app install CPU share : change default:

I just think it is bizarre / bad practice to set the default to 50%.

Why? have you actually hit a problem of some some sort that requires a solution?

@timconsidine said in New app install CPU share : change default:

If it doesn't have a practical impact, why bother having it

Fair question, i guess.

@timconsidine said in New app install CPU share : change default:

Anyway, it doesn't really matter, I guess.

My thoughts exactly.

I think the only time I've ever adjusted mine has been to give Nextcloud even more than 50% (I've given it 75%) in the hope it might help speed it up.

Oh, and on one of my PeerTube instance too (a transcoding videos seems to one of the few things I do on my server that actually eats up a fair bit of CPU).

@humptydumpty said in New app install CPU share : change default:

The graph shows usage/spikes over 100% sometimes which doesn’t make sense at all

That's when there are multiple CPUs (which there normally are). My server seemingly has 12 CPUs as my graph goes up to 1200% (but in the last 30 days it's never actually gone above 225%)

[jdaviescoates]

@girish said in New app install CPU share : change default:

I tried to summarize the above in https://docs.cloudron.io/apps/#cpu-shares (screenshot) but maybe it's not clear enough.

You've written there:

The CPU share setting is a percentage relative to each app

Does that mean that if one leaves all the apps on 50% the setting is basically doing nothing? But that any more/ less than 50% would give relatively more/ less CPU time to that app?

[girish]

@jdaviescoates yes, that's my understanding. setting it all to 50% means you are not giving any app any "preference".