First cut at least cost scheduler

2 files changed, 118 insertions, 0 deletions

diff --git a/nova/scheduler/least_cost.py b/nova/scheduler/least_cost.py
new file mode 100644
index 000000000..75dde81ca
--- /dev/null
+++ b/nova/scheduler/least_cost.py
@@ -0,0 +1,79 @@
+import collections
+
+# TODO(sirp): this should be just `zone_aware` to match naming scheme
+# TODO(sirp): perhaps all zone-aware stuff should go under a `zone_aware`
+# module
+from nova.scheduler import zone_aware_scheduler
+
+class LeastCostScheduler(zone_aware_scheduler.ZoneAwareScheduler):
+    def get_cost_fns(self):
+        """Returns a list of tuples containing weights and cost functions to
+        use for weighing hosts
+        """
+        cost_fns = []
+
+        return cost_fns
+
+    def weigh_hosts(self, num, specs, hosts):
+        """
+        Returns a list of dictionaries of form:
+            [ {weight: weight, hostname: hostname} ]
+        """
+        # FIXME(sirp): weigh_hosts should handle more than just instances
+        cost_fns = []
+        hosts = []
+        cost_hosts = weighted_sum(domain=hosts, weighted_fns=self.get_cost_fns())
+
+        # TODO convert hosts back to hostnames
+        weight_hostnames = []
+        return weight_hostnames 
+
+def normalize_list(L):
+    """Normalize an array of numbers such that each element satisfies:
+        0 <= e <= 1
+    """
+    if not L:
+        return L
+    max_ = max(L)
+    if max_ > 0:
+        return [(float(e) / max_) for e in L]
+    return L
+
+def weighted_sum(domain, weighted_fns, normalize=True):
+    """
+    Use the weighted-sum method to compute a score for an array of objects.
+    Normalize the results of the objective-functions so that the weights are
+    meaningful regardless of objective-function's range.
+    
+    domain - input to be scored
+    weighted_fns - list of weights and functions like:
+        [(weight, objective-functions)]
+
+    Returns an unsorted list like: [(score, elem)]
+    """
+    # Table of form:
+    #   { domain1: [score1, score2, ..., scoreM]
+    #     ...
+    #     domainN: [score1, score2, ..., scoreM] }
+    score_table = collections.defaultdict(list)
+
+    for weight, fn in weighted_fns:
+        scores = [fn(elem) for elem in domain]
+
+        if normalize:
+            norm_scores = normalize_list(scores)
+        else:
+            norm_scores = scores
+
+        for idx, score in enumerate(norm_scores):
+            weighted_score = score * weight
+            score_table[idx].append(weighted_score)
+
+    # Sum rows in table to compute score for each element in domain
+    domain_scores = []
+    for idx in sorted(score_table):
+        elem_score = sum(score_table[idx])
+        elem = domain[idx]
+        domain_scores.append(elem_score)
+
+    return domain_scores
diff --git a/nova/tests/test_least_cost_scheduler.py b/nova/tests/test_least_cost_scheduler.py
new file mode 100644
index 000000000..a3a18a09f
--- /dev/null
+++ b/nova/tests/test_least_cost_scheduler.py
@@ -0,0 +1,39 @@
+from nova import test
+from nova.scheduler import least_cost
+
+MB = 1024 * 1024
+
+class FakeHost(object):
+    def __init__(self, host_id, free_ram, io):
+        self.id = host_id
+        self.free_ram = free_ram
+        self.io = io
+
+class WeightedSumTest(test.TestCase):
+    def test_empty_domain(self):
+        domain = []
+        weighted_fns = []
+        result = least_cost.weighted_sum(domain, weighted_fns)
+        expected = []
+        self.assertEqual(expected, result)
+
+    def test_basic_costing(self):
+        hosts = [
+            FakeHost(1, 512 * MB, 100),
+            FakeHost(2, 256 * MB, 400),
+            FakeHost(3, 512 * MB, 100)
+        ]
+
+        weighted_fns = [
+            (1, lambda h: h.free_ram),  # Fill-first, free_ram is a *cost*
+            (2, lambda h: h.io),  # Avoid high I/O
+        ]
+
+        costs = least_cost.weighted_sum(domain=hosts, weighted_fns=weighted_fns)
+
+        # Each 256 MB unit of free-ram contributes 0.5 points by way of:
+        #   cost = weight * (score/max_score) = 1 * (256/512) = 0.5
+        # Each 100 iops of IO adds 0.5 points by way of:
+        #   cost = 2 * (100/400) = 2 * 0.25 = 0.5
+        expected = [1.5, 2.5, 1.5]
+        self.assertEqual(expected, costs)