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Re: [HTCondor-users] htcondor cgroups and memory limits on CentOS7

On 10/20/2017 9:44 AM, Alessandra Forti wrote:

is more information needed?

Hi Alessandra,

The version of HTCondor you are using would be helpful :).

But I have some answers/suggestions below that I hope will help...

* On the head node

RemoveMemoryUsage = ( ResidentSetSize_RAW > 2000*RequestMemory )
SYSTEM_PERIODIC_REMOVE = $(RemoveMemoryUsage)Â || <OtherParameters>

So the questions are two


Because the (system_)periodic_remove expressions are evaluated by the condor_shadow while the job is running, and the *_RAW attributes are only updated in the condor_schedd.

A simple solution is to use attribute MemoryUsage instead of ResidentSetSize_RAW. So I think things will work as you want if you instead did:

  RemoveMemoryUsage = ( MemoryUsage > 2*RequestMemory )
  SYSTEM_PERIODIC_REMOVE = $(RemoveMemoryUsage)  || <OtherParameters>

Note that MemoryUsage is in the same units as RequestMemory, so only need to multiply by 2 instead of 2000.

You are not the first person to be tripped up by this. :( I realize it is not at all intuitive. I think I will add a quick patch in the code to allow _RAW attributes to be referenced inside of job policy expressions to help prevent frustration by the next person.

Also you may want to place your memory limit policy on the execute nodes via startd policy expression, instead of having them enforced on the submit machine (what I think you are calling the head node). The reason is the execute node policy is evaluated every five seconds, while the submit machine policy is evaluated every several minutes. A runaway job could consume a lot of memory in a few minutes :).

2) Shouldn't htcondor set the job soft limit with this configuration? or is the site expected to set the soft limit separately?

Personally, I think "soft" limits in cgroups are completely bogus. The way the Linux kernel treats soft limits does not do in practice what anyone (including htcondor itself) expects. I recommend settings CGROUP_MEMORY_LIMIT to either none or hard, soft makes no sense imho.

"CGROUP_MEMORY_LIMIT=hard" is clear to understand: if the job uses more memory than it requested, it is __immediately__ kicked off and put on hold. This way users get a consistent experience.

If you want jobs to be able to go over their requested memory so long as the machine isn't swapping, consider disabling swap on your execute nodes (not a bad idea for compute servers in general) and simply leaving "CGROUP_MEMORY_LIMIT=none". What will happen is if the system is stressed, eventually the Linux OOM (out of memory killer) will kick in and pick a process to kill. HTCondor sets the OOM priority of job process such that the OOM killer should always pick job processes ahead of other processes on the system. Furthermore, HTCondor "captures" the OOM request to kill a job and only allows it to continue if the job is indeed using more memory than requested (i.e. provisioned in the slot). This is probably what you wanted by setting the limit to soft in the first place.

I am thinking we should remove the "soft" option to CGROUP_MEMORY_LIMIT in future releases, it just causes confusion imho. Curious if others on the list disagree...

Hope the above helps,