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

[Dario Faggioli <dfaggioli@suse.com>]

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On Wed, 2022-05-18 at 12:17 +0200, Igor Mammedov wrote:
> On Tue, 17 May 2022 20:46:50 +0200
> Paolo Bonzini <pbonzini@redhat.com> wrote:
> > > diff --git a/backends/hostmem.c b/backends/hostmem.c
> > > index a7bae3d713..624bb7ecd3 100644
> > > --- a/backends/hostmem.c
> > > +++ b/backends/hostmem.c
> > > @@ -274,7 +274,7 @@ static void host_memory_backend_init(Object
> > > *obj)
> > >       backend->merge = machine_mem_merge(machine);
> > >       backend->dump = machine_dump_guest_core(machine);
> > >       backend->reserve = true;
> > > -    backend->prealloc_threads = 1;
> > > +    backend->prealloc_threads = machine->smp.cpus;
> > >   }
> > >   
> > >   static void host_memory_backend_post_init(Object *obj)  
> > 
> > Queued, thanks.
> 
> PS:
> There is no good default in this case (whatever number is picked
> it could be good or bad depending on usecase).
> 
That is fair enough. What we observed, however, is that, with QEMU 5.2,
starting a 1024G VM takes ~34s.

Then you just update QEMU to > 5.2 (and don't do/changing anything
else) and the same VM now takes ~4m30s to start.

If users are managing QEMU via Libvirt *and* have _at_least_ Libvirt
8.2, they can indeed set, e.g., <allocation mode='immediate'
threads='NNN'/> (provided they can understand where the problem is, and
figure out that this is the solution).

If they have Libvirt < 8.2 (e.g., people/distros that have, say, QEMU
6.2 and Libvirt 8.0.0, or something like that), there's basically
nothing they can do... Except perhaps command line passthrough [1], but
that's really rather tricky!

So, I personally don't know where any default should be set and how,
but the above situation is not nice for users to have to handle.

[1] https://libvirt.org/kbase/qemu-passthrough-security.html

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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