Re: [Qemu-devel] [RFCv2 2/2] spapr: Don't use QOM [*] syntax for DR connectors.

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

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 <david@gibson.dropbear.id.au>
> Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>

This patch works correctly with both CPU and memory hotplug.

Regards,
Bharata.