Re: [Libvir] PATCH: Improve performance on Xen

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.