Comment 3 for bug 1908805

The cause for the high RSS even when idle is a high water mark memory issue caused by the per native thread glibc malloc arena.

I'm proposing a devstack patch to fix it https://review.opendev.org/c/openstack/devstack/+/845805 and a tripleo one as well https://review.opendev.org/c/openstack/tripleo-common/+/845807

You can try it in your deployment yourself running the cinder-backup service with those environmental variables.