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

[Daniel P. Berrange]

On Wed, Jan 04, 2017 at 11:01:30AM -0500, Programmingkid wrote:
> 
> On Jan 4, 2017, at 10:51 AM, Daniel P. Berrange wrote:
> 
> > 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.
> 
> Are you saying that so we can use your Virt-manager? The thing is having more
> options is a good thing. If one solution doesn't work, another solution might.
> We don't live in a world with only one Linux distribution. We live in a world
> with many Linux distributions. We have the power of choice. There currently
> isn't a single QEMU front-end that runs on all three operating systems. Yes 
> the
> Linux people do have Virt-manager, but what about the Windows and Macintosh
> people. They are not so fortunate. Enhancing the GTK front-end does not
> hurt any of the Virt-manager users at all. This change is just for the people
> who prefer the GTK front-end.

This is not about choice, it is about the focusing QEMU community resources
where they are most effectively applied. Turning the GTK front-end into a
fully featured mgmt UI would put a significant, ongoing burden on the QEMU
maintainers over the long term. This will taking resources away from other
areas of work in QEMU, just to duplicate features that already exist / will
exist in layers above / apps external to QEMU.

Regards,
Daniel
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