scheduler.py 1.95 KB
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from django.db.models import Sum


class NotEnoughMemoryException(Exception):
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    def __init__(self, message=None):
        if message is None:
            message = "No node has enough memory to accomodate the guest."

        Exception.__init__(self, message)


class TraitsUnsatisfiableException(Exception):
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    def __init__(self, message=None):
        if message is None:
            message = "No node can satisfy all required traits of the guest."

        Exception.__init__(self, message)


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def select_node(instance, nodes):
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    ''' Select a node for hosting an instance based on its requirements.
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    '''
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    # check required traits
    nodes = [n for n in nodes if has_traits(instance.req_traits.all(), n)]
    if not nodes:
        raise TraitsUnsatisfiableException()

    # check required RAM
    nodes = [n for n in nodes if has_enough_ram(instance.ram_size, n)]
    if not nodes:
        raise NotEnoughMemoryException()

    # sort nodes first by processor usage, then priority
    nodes.sort(key=lambda n: n.priority, reverse=True)
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    nodes.sort(key=free_cpu_time, reverse=True)
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    return nodes[0]


def has_traits(traits, node):
    """True, if the node has all specified traits; otherwise, false.
    """
    traits = set(traits)
    return traits.issubset(node.traits.all())


def has_enough_ram(ram_size, node):
    """True, if the node has enough memory to accomodate a guest requiring
       ram_size mebibytes of memory; otherwise, false.
    """
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    unused = node.ram_size * (1 - node.ram_usage)
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    overcommit = node.ram_size_with_overcommit
    reserved = node.instance_set.aggregate(r=Sum('ram_size'))['r'] or 0
    free = overcommit - reserved

    return ram_size < unused and ram_size < free


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def free_cpu_time(node):
    """Get an indicator number for idle processor time on the node.
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    Higher values indicate more idle time.
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    """
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    activity = node.cpu_usage
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    inactivity = 1 - activity
    cores = node.num_cores
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    return cores * inactivity