This issue is happening in Carbon/Nitrogen (not in Boron), it is tracked here:

To reproduce:

1) Start mininet linear 32:
sudo mn --controller 'remote,ip=192.168.0.1,port=6633' --topo linear,32

2) Push 100K flows (script available in int/test repo):
python odl_tester.py --threads 5 --flows 100000 --no-delete --fpr 200

Observe after 100K flows are added and controller is stable CPU is still very high.

3) Remove flows from inventory:
DELETE http://controller:8181/restconf/config/opendaylight-inventory:nodes/

Observe after 100K flows are removed and controller is stable, there are still flows in operational that never get removed.

CSIT job for this test : ttps://jenkins.opendaylight.org/releng/job/openflowplugin-csit-1node-periodic-scale-stats-collection-daily-only-carbon/