[libvirt] increase daemon task limit

[Daniel P. Berrange]

On Wed, Apr 19, 2017 at 09:03:55AM -0600, Jim Fehlig wrote:
> I recently received a report of a libvirt+Xen installation reaching the pids
> cgroup controller TasksMax limit
> 
> kernel: [71282.213347] cgroup: fork rejected by pids controller in
> /system.slice/libvirtd.service
> 
> The default setting of TaskMax is 512 on this system
> 
> # cat /sys/fs/cgroup/pids/system.slice/libvirtd.service/pids.max
> 512
> 
> Depending on domain type and configuration, I've noticed between 5-7 tasks
> (IO threads, qemu process, etc.) are created when starting a domain. It
> doesn't take too many domains before the 512 limit is reached.
> 
> LimitNOFILE was recently changed by commit 27cd763500 to support 4096
> domains. Following similar logic, would it be ok to increase TasksMax to
> 32768? That would accommodate 4096 domains with 8 tasks each. TasksMax also
> supports the special value of "infinity", but that seems a bit aggressive to
> me.

KVM uses 1 thread per vCPU, so 8 pids is probably somewhat on the low
side if considering high vcpu counts. I think we could go for something
like 128k pids which allows for closer to 32 vcpus per guest


Regards,
Daniel
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