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Re: qemu modularization of qemu-5.1 vs libvirt domcapabilities cache?

On Thu, Aug 20, 2020 at 11:32:03AM +0200, Martin Wilck wrote:
> On Tue, 2020-08-18 at 15:15 -0600, Jim Fehlig wrote:
> > On 8/5/20 2:19 AM, Andrea Bolognani wrote:
> > > 
> > > I guess we need to start checking the modules directory in addition
> > > to the main QEMU binary, and regenerate capabilities every time its
> > > contents change.
> > 
> > We recently received reports of this issue on Tumbleweed, which just
> > got the 
> > modularized qemu 5.1
> > 
> > https://bugzilla.opensuse.org/show_bug.cgi?id=1175320
> > 
> > Mark, are you working on a patch to invalidate the cache on changes
> > to the qemu 
> > modules directory? I suppose it needs to be handled similar to the
> > qemu 
> > binaries. E.g. when building the cache include a list of all qemu
> > modules found. 
> > When validating the cache see if any modules have disappeared, if any
> > have been 
> > added, and if the ctime of any have changed. Yikes, sounds a little
> > more complex 
> > than the binaries :-).
> I'd like to question whether this effort is justified for an
> optimization that matters only at libvirtd startup time, and even there
> saves no more than a few seconds.
> I propose to simply disable the caching of qemu capabilities (or
> provide a build-time option to do so). Optimizations that produce wrong
> results should be avoided.

Whether the time matters depends on your use case for QEMU. For heavy
data center apps like OpenStack you won't notice it because OpenStack
itself adds soo much overhead to the system.  For cases where the VM
is used as "embedded" infrastructure startup time can be critical.
Not caching capabilities adds easily 300-500 ms to startup of a single
VM, which is very significant when the current minimum startup time of
a VM can be as low as 150 ms.

IOW removing caching is not a viable option.

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