[Libvir] PATCH: Improve performance on Xen

[Daniel Veillard]

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.

> 
> Any for the 3 improvements I have done, the 'virsh dominfo' call has improved
> when using either ID or UUID:
[...]
> A pretty good speedup really considering how frequently these calls are
> made if you're monitoring domains frequently.

  Too good to ignore, that looks even too good to be true :-)

> Index: src/xen_internal.c

  Patch looks good to me +1, fanxastic !

Daniel

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