Also: the 524 threads was with Xen PVM and two VCPUS. With one VCPU the problem goes away. The run on 4.8.0-22-generic also had two VCPUs.
There is no problem with Xen HVM and 4.8.0-26-generic with either one or two VCPUS.
Also: the 524 threads was with Xen PVM and two VCPUS. With one VCPU the problem goes away. The run on 4.8.0-22-generic also had two VCPUs.
There is no problem with Xen HVM and 4.8.0-26-generic with either one or two VCPUS.