[Qemu-devel] [RFC PATCH] spapr: Reduce creation of LMB DR connectors from O(n^3) to O(n^2)

[David Gibson <david@gibson.dropbear.id.au>]

The dynamic reconfiguration (hotplug) code for the pseries machine type
uses a "DR connector" QOM object for each resource it will be possible
to hotplug.  Each of these is added to its owner using
    object_property_add_child(owner, "dr-connector[*], ...);

This works ok for most cases, but gets ugly when allowing large amounts of
hotplugged RAM.  For RAM, there's a DR connector object for every 256MB of
potential memory.  So if maxmem=2T, for example, there are >250,000 objects
under the same parent.

The QOM interfaces aren't really designed for this.  In particular
object_property_add() has O(n^2) time complexity (in the number of existing
children) for the [*] case.  First it has a linear search through array
indices to find a free slot, each of which is attempted to a recursive call
to object_property_add() with a specific [N].  Those calls are O(n) because
there's a linear search through all properties to check for duplicates.

For the specific case of DR connectors, we already have a sufficiently
unique index, so we don't need to use the [*] special behaviour.  That lets
us reduce the total time for creating the DR objects from O(n^3) to O(n^2).

O(n^2) is still kind of crappy, but it's enough to reduce the startup time
of qemu with maxmem=2T from ~20 minutes to ~4 seconds.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
 hw/ppc/spapr_drc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
index c1f664f..4cf3a9b 100644
--- a/hw/ppc/spapr_drc.c
+++ b/hw/ppc/spapr_drc.c
@@ -463,14 +463,16 @@ sPAPRDRConnector *spapr_dr_connector_new(Object *owner,
 {
     sPAPRDRConnector *drc =
         SPAPR_DR_CONNECTOR(object_new(TYPE_SPAPR_DR_CONNECTOR));
+    char *prop_name = g_strdup_printf("dr-connector[%"PRIu32"]", id);
 
     g_assert(type);
 
     drc->type = type;
     drc->id = id;
     drc->owner = owner;
-    object_property_add_child(owner, "dr-connector[*]", OBJECT(drc), NULL);
+    object_property_add_child(owner, prop_name, OBJECT(drc), NULL);
     object_property_set_bool(OBJECT(drc), true, "realized", NULL);
+    g_free(prop_name);
 
     /* human-readable name for a DRC to encode into the DT
      * description. this is mainly only used within a guest in place
-- 
2.4.3