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Re: [libvirt] [PATCH] RFC: Remove all object hashtable caches from virConnectPtr



On Fri, Oct 29, 2010 at 12:52:18PM +0100, Daniel P. Berrange wrote:
> The virConnectPtr struct will cache instances of all other
> objects. APIs like virDomainLookupByUUID will return a
> cached object, so if you do virDomainLookupByUUID twice in
> a row, you'll get the same exact virDomainPtr instance.
> 
> This does not have any performance benefit, since the actual
> logic in virDomainLookupByUUID (and other APIs returning
> virDomainPtr, etc instances) is not short-circuited. All
> it does is to ensure there is only one single virDomainPtr
> in existance for any given UUID.
> 
> The caching has a number of downsides though, all relating
> to stale data. If APIs aren't careful to always overwrite
> the 'id' field in virDomainPtr it may become out of data.
> Likewise for the name field, if a guest is renamed, or if
> a guest is deleted, and then a new one created with the
> same UUID but different name.
> 
> This has been an ongoing, endless source of bugs for all
> applications using libvirt from languages with garbage
> collection, causing them to get virDomainPtr instances
> from long ago with stale data.
> 
> The caching is also a waste of memory resources, since
> both applications, and language bindings often maintain
> their own hashtable caches of object instances.
> 
> This patch removes all the hash table caching, so all
> APIs return brand new virDomainPtr (etc) object instances.

No one has any comments on this change I thought would be
hugely controversial... ?

Daniel
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