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



On Thu, Nov 11, 2010 at 11:44:42AM -0500, Dave Allan wrote:
> On Thu, Nov 11, 2010 at 04:20:43PM +0000, Daniel P. Berrange wrote:
> > 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... ?
> 
> From what you've decribed, it seems like a good change.  I would have
> thought that there would be a performance hit, but from what you've
> said that's not the case.  Why did you think it would be
> controversial?

Technically it is a semantic API change, because you can no longer rely
on two virDomainPtr for the same domain having the same pointer address.
I think this is pretty unlikely to hurt anything, and any app relying
on this can be argued to be broken, but you never know....

Daniel
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