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Re: [Libvir] PATCH: Improve performance on Xen



On Fri, Aug 10, 2007 at 06:39:56AM -0400, Daniel Veillard wrote:
> On Thu, Aug 09, 2007 at 11:13:00PM +0100, Daniel P. Berrange wrote:
> > The Xen implementations of
> > 
> >    virDomainLookupByID
> >    virDomainLookupByUUID
> >    virDomainLookupByName
> >    virDomainGetOSType
> > 
> > all have sub-optimal performance since they speak to XenD. The lookupXXX
> > functions all basically require 3 pieces of info in the end (name,id,uuid).
> > They each get given one piece & have to lookup the other piece.
> > 
> > Every running domain has to have an entry in XenStore on /local/domain/N
> > where 'N' is the id. Under this location there is always a 'name' field.
> > So if we have the id we can efficiently get the name. I just realized that
> > the getdomaininfo hypercall struct contains the uuid and id. So for the
> > ByID and ByUUID calls we can get all the info we need with a hypercall and
> > a read of xenstore. This is very efficient compared to hitting XenD.
> 
>   Great !!!!
> 
> > As of Xen 3.1.0 hypervisor the flags in the getdomaininfo hypercall struct
> > also mark whether the guest is HVM vs paraivrt, so we can also implement
> > the GetOSType api with a single hypercall.
> 
>   Sounds good to though definitely not a bottleneck.
> 
> > That just leaves the ByName impl. The only way I can think of doing this
> > is to scan /local/domain/N  in xenstore until we find it. I'm going to try
> > this, but I'm not convinced it'll be any significantly than talking to XenD.
> > I may be surprised though.
> 
>   Well if you have 100 guests, that may be slower, but in the average situation
> of only a couple of guests, it could be a real speedup. The problem is that
> a lot of domain may accumulate in xenstore /local/domain even if they are
> not running, both implementation are likely to have completely different 
> behaviour based on the context. But from a cache locality perspective hitting
> xenstore may scale way better under loaded machines, so it may prove faster
> even on machines with hundreds of domains. Doing a fair performance comparison
> may prove really hard.

Actually the /local/domain/[ID] subtree is guarenteed to only contain running 
VMs. The /vm/[UUID] subtree is where cruft accumulates over time, so its safe 
to rely on info in the former, but not the latter

Dan.
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