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

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

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On Mon, Sep 14, 2015 at 09:37:16AM +0530, Bharata B Rao wrote:
> 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.

Care to send a Reviewed-by and/or Tested-by in that case?

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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