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# Copyright (c) 2016, Red Hat Inc.
# All Rights Reserved.
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations
# under the License.
CPU Weigher. Weigh hosts by their CPU usage.
The default is to spread instances across all hosts evenly. If you prefer
stacking, you can set the 'cpu_weight_multiplier' option (by configuration
or aggregate metadata) to a negative number and the weighing has the opposite
effect of the default.
import nova.conf
from nova.scheduler import utils
from nova.scheduler import weights
CONF = nova.conf.CONF
class CPUWeigher(weights.BaseHostWeigher):
minval = 0
def weight_multiplier(self, host_state):
"""Override the weight multiplier."""
return utils.get_weight_multiplier(
host_state, 'cpu_weight_multiplier',
def _weigh_object(self, host_state, weight_properties):
"""Higher weights win. We want spreading to be the default."""
vcpus_free = (
host_state.vcpus_total * host_state.cpu_allocation_ratio -
return vcpus_free