Re: [PATCH] hostmem: default the amount of prealloc-threads to smp-cpus

[Paolo Bonzini <pbonzini@redhat.com>]

On 5/18/22 15:31, Daniel P. Berrangé wrote:
> When picking defaults there is never a perfect answer, it
> is more a matter of the least-worst option.
> 
> It is pretty clear that nthreads=1 is terrible for any
> large VMs. Defaulting it to nvcpus made conceptual sense
> as the user has implicit said that they expect the VM to
> be able to consume nvcpus worth of CPU time on the host,
> so we might as well consume that allotted resource.

I agree.  Yes, one could argue that the regression was on the libvirt 
side, but it's easier to fix it in QEMU.

If we later add the ability to create a memory backend before machine 
creation (for example with a QMP-only binary), then it's of course okay 
for those backends to use only one thread and require a manual choice 
for the # or preallocation threads.

Paolo