[Libvir] Scability / performance fix for virDomainLookupByID

[Daniel Veillard]

On Fri, Jul 07, 2006 at 03:47:59PM +0100, Daniel P. Berrange wrote:
> Attached is a patch to significantly increase scalability / performance of 
> the xenDaemonLookupByID method. The current implementation would get a
> list of all domain names from XenD, and then iterate doing a HTTP GET on
> /xend/domain/[name] until the domain with match ID was found. THis had
> O(n) complexity, with the result that when running on a system with 20
> actives domains, 'virsh list' would have O(n^2) complexity needing ~230  
> HTTP calls, giving a runtime of ~9 seconds.
> 
> The patch is to make the code do a HTTP GET on /xend/domain/[id] which we
> just discovered is a valid URL to access. This makes the method call O(1),

  I should have guessed that earlier, especially after the report
last week about virDomainLookupByName(conn, "1") working fine ...

> and 'virsh list' is now a saner O(n), and completes in ~1 second. While 
> still not great performance, this is certainly much better. I think it
> ought to be possible to optimize the code still further so that XenD is
> avoided altogether for simple commands which can be fullfilled purely
> with data available from Hypervisor, but that will need further 
> investigation. 
> 
> Please review the patch in case I missed any bugs / edge cases

  just 2 small things:

in xenDaemonLookupByID, it seems (but I may have misread the patch)
that the free of name in the error code in case the rpc failed is a bit
risky and should be guarded by if (name != NULL)

and the documentation for the new function in xend_internal.h is the old
one inherited from Anthony first version, I somehow deprecated it, documenting
the function itself in the .c file, but it's just nitpicking :-)

  thanks a lot for finding this ! Please apply :-)

Daniel

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