On Mon, Sep 14, 2015 at 11:41:53AM +1000, David Gibson wrote: > 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[*], ...); > > That works ok, mostly, but it means that the property indices are > arbitrary, depending on the order in which the connectors are constructed. > When we have both memory and cpu hotplug, the connectors will be under the > same parent (at least in the current drafts), meaning the indices don't > correspond to any meaningful ID. > > It gets worse when large amounts of hotpluggable RAM is configured. For > RAM, there's a DR connector object for every 256MB of potential memory. So > if maxmem=2T, for example, there are 8192 objects under the same parent. > > The QOM interfaces aren't really designed for this. In particular > object_property_add() with [*] has O(n^2) time complexity (in the number of > existing children): 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. > > By using a meaningful index value, which we already know is unique we can > avoid 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 <da...@gibson.dropbear.id.au> > Cc: Bharata B Rao <bhar...@linux.vnet.ibm.com>
This patch works correctly with both CPU and memory hotplug. Regards, Bharata.
