perf(computes): a host's capacity is one libvirt call, not one per VM

The Computes page, the host overview and the new-VM dialog's capacity line
read each VM's XML for its memory and vCPUs: a round trip a VM, about
200 ms for 150 VMs on a LAN, more with each host. libvirt's domain stats
give them for every VM in one call; a running VM busy with a job is not
waited for (NOWAIT) and its sizes come from its XML as before.

vrtManager/hostdetails.py

@@ -2,12 +2,17 @@ import time
 
 from libvirt import (
     libvirtError,
+    VIR_CONNECT_GET_ALL_DOMAINS_STATS_NOWAIT,
     VIR_DOMAIN_BLOCKED,
     VIR_DOMAIN_PAUSED,
     VIR_DOMAIN_RUNNING,
     VIR_DOMAIN_SHUTOFF,
+    VIR_DOMAIN_STATS_BALLOON,
+    VIR_DOMAIN_STATS_STATE,
+    VIR_DOMAIN_STATS_VCPU,
     VIR_NODE_CPU_STATS_ALL_CPUS,
 )
+from lxml import etree
 
 from vrtManager.connection import wvmConnect
 from vrtManager.util import get_xml_path
@@ -111,8 +116,7 @@ class wvmHostDetails(wvmConnect):
         vcpus = {"all": 0, "running": 0}
         assigned = {"all": 0, "running": 0}
         vms = {"running": 0, "paused": 0, "shutoff": 0, "other": 0}
-        # the VM lists' read: the states in one call, each VM's XML once
-        for (state, _max_memory, memory, vcpu), _title in self.get_domain_rows().values():
+        for state, memory, vcpu in self._domain_sizes():
             for share in ("all",) if state == VIR_DOMAIN_SHUTOFF else ("all", "running"):
                 vcpus[share] += vcpu
                 assigned[share] += memory * 1024
@@ -120,6 +124,22 @@ class wvmHostDetails(wvmConnect):
         return {"cpus": self.host_info[2], "memory": self.host_info[1] * 1048576,
                 "vcpus": vcpus, "assigned": assigned, "vms": vms}
 
+    def _domain_sizes(self):
+        """Each VM's state, memory (KiB) and vCPUs, in one call: not its XML,
+        one round trip a VM. A running VM busy with a job (a migration, a
+        hung QEMU) is not waited for: its sizes come from its XML."""
+        flags = VIR_DOMAIN_STATS_STATE | VIR_DOMAIN_STATS_BALLOON | VIR_DOMAIN_STATS_VCPU
+        for dom, stats in self.wvm.getAllDomainStats(flags, VIR_CONNECT_GET_ALL_DOMAINS_STATS_NOWAIT):
+            memory, vcpu = stats.get("balloon.current"), stats.get("vcpu.current")
+            if memory is None or vcpu is None:
+                try:
+                    doc = etree.fromstring(dom.XMLDesc(0))
+                except libvirtError:
+                    continue  # gone meanwhile
+                memory, vcpu = int(doc.findtext("currentMemory")), doc.find("vcpu")
+                vcpu = int(vcpu.get("current", vcpu.text))
+            yield stats["state.state"], memory, vcpu
+
     def get_cpu_topology(self):
         """
         The host's logical CPUs and MHz, and from the CPUs of its NUMA cells

vrtManager/tests/test_host_reads.py

@@ -158,19 +158,24 @@ class CapacityTestCase(unittest.TestCase):
         conn = connection(wvmHostDetails)
         gib = 1024 ** 3
         kib = gib // 1024
-        # get_domain_rows: info (state, max memory KiB, memory KiB, vCPUs) and title
-        conn.get_domain_rows = MagicMock(return_value={
-            "u-web": ([VIR_DOMAIN_RUNNING, 8 * kib, 4 * kib, 4], ""),
-            "u-db": ([VIR_DOMAIN_PAUSED, 2 * kib, 2 * kib, 2], ""),
-            "u-old": ([VIR_DOMAIN_SHUTOFF, 8 * kib, 8 * kib, 8], ""),
-        })
+        busy = MagicMock(**{"XMLDesc.return_value": f"<domain><currentMemory>{kib}</currentMemory>"
+                                                      "<vcpu current='1'>4</vcpu></domain>"})
+        idle = MagicMock()
+        # one call for every VM; a VM busy with a job has only its state: its XML gives the rest
+        conn.wvm.getAllDomainStats = MagicMock(return_value=[
+            (idle, {"state.state": VIR_DOMAIN_RUNNING, "balloon.current": 4 * kib, "vcpu.current": 4}),
+            (idle, {"state.state": VIR_DOMAIN_PAUSED, "balloon.current": 2 * kib, "vcpu.current": 2}),
+            (idle, {"state.state": VIR_DOMAIN_SHUTOFF, "balloon.current": 8 * kib, "vcpu.current": 8}),
+            (busy, {"state.state": VIR_DOMAIN_RUNNING}),
+        ])
         self.assertEqual(conn.get_capacity(), {
             "cpus": 4, "memory": 2 * gib,
-            "vcpus": {"all": 14, "running": 6},
-            "assigned": {"all": 14 * gib, "running": 6 * gib},
-            "vms": {"running": 1, "paused": 1, "shutoff": 1, "other": 0},
+            "vcpus": {"all": 15, "running": 7},
+            "assigned": {"all": 15 * gib, "running": 7 * gib},
+            "vms": {"running": 2, "paused": 1, "shutoff": 1, "other": 0},
         })
-        conn.get_domain_rows.assert_called_once_with()
+        self.assertEqual(conn.wvm.getAllDomainStats.call_count, 1)
+        idle.XMLDesc.assert_not_called()
 
 
 class CpuTopologyTestCase(unittest.TestCase):