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

[Gerd Hoffmann <kraxel@redhat.com>]

  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%".

cheers,
  Gerd