Re: [Qemu-devel] Speed menu for GTK interfaace

[Daniel P. Berrange]

On Wed, Jan 04, 2017 at 09:37:39AM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > It is quite simple, there would be a 100% to a 1% menu item. It would look 
> > like
> > this:
> > 
> > Speed
> > -------
> > 100%
> > 90%
> > 80%
> > 70%
> > 60%
> > 50%
> > 40%
> > 30%
> > 20%
> > 10%
> > 1%
> > 
> > 
> > Each menu item would call cpu_throttle_set(). The value sent to this 
> > function would
> > be determined like this:
> > speed = -1 * menu_number + 100;
> 
> ok, that is the info I was looking for.
> 
> > speed would be sent to the cpu_throttle_set() function. This function would 
> > reduce
> > the CPU usage of QEMU on the host. 
> > 
> > Why would someone want to slow down QEMU?
> > - The user is using a laptop and don't want it to heat up.
> 
> Sort-of makes sense, to keep the laptop quiet.
> 
> > - The user wants to slow down a video game that is a little too challenging.
> 
> Sure this would work?  Throttling isn't a smooth slowdown, the cpu
> continues to run at full speed and is forced to pause now and then.
> 
> > - The user wants to save energy.
> 
> Pointless.  Laptop may run longer, but your job needs more time to
> complete too.  And constant vcpu start/stop isn't good to save power,
> the cpus can't enter deep sleep states then because of the frequent
> wakeups.
> 
> > - The user wants to conduct some kind of stress test on a program and see 
> > how
> > it handles under low cpu resources. 
> 
> Makes sense too.
> 
> We already have "pause" in gtk, adding a "throttle" item next to it
> looks reasonable to me.  I don't think it is that useful to have 10%
> steps in there, you probably never throttle 10% in practice.  It's
> probably more useful to have something like "throttle -> off / 50% /
> 90% / 95% / 99%".

This feels like going down the slippery slope to turn the GTK frontend
into a full mgmt UI, which is something we've said we don't want todo
inside QEMU UI frontends. IMHO this kind of feature is best left to
external mgmt layers, like  virt-manager/GNOME Boxes/etc. It is already
possible to timebox VMs CPU execution using cgroups quotas via libvirt.

Regards,
Daniel
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