Re: [Xen-devel] Implement 3-level event channel routines in Linux.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Jan 03, 2013 at 12:09:25PM +0000, Wei Liu wrote:
> On Wed, 2013-01-02 at 21:12 +0000, Konrad Rzeszutek Wilk wrote:
> > On Wed, Jan 02, 2013 at 06:46:58PM +0000, Wei Liu wrote:
> > > On Wed, 2013-01-02 at 18:26 +0000, Konrad Rzeszutek Wilk wrote:
> > > > On Mon, Dec 31, 2012 at 06:38:54PM +0000, Wei Liu wrote:
> > > > > This patch series implements 3-level event channel routines in Linux 
> > > > > kernel.
> > > > > 
> > > > > My thought is that 3-level event channel is only useful for Dom0 or 
> > > > > driver
> > > > > domain, so it is not enabled by default. Enable it with 
> > > > > evtchn_level=3 in
> > > > > kernel command line.
> > > > 
> > > > Could it be enabled dynamically? Say when we are close to exhausting the
> > > > amount of channels? Or if the machine has some large amount of memory 
> > > > and
> > > > hence would probably allocate many many guests?
> > > > 
> > > 
> > > Do you mean enabling this mechanism on the fly when Dom0 / driver domain
> > > is running? Or do you mean enabling it based on some metrics when
> > > starting up a domain?
> > 
> > Either one.
> > > 
> > > If it is the first case, who is responsible for initiating the switching
> > > process? It looks like it is Xen's responsibility to do the switch
> > > because Dom0 sometimes cannot know the true capability of underlying
> > > hardware. If it is Xen, then we need to add callback to poke Dom0 /
> > > driver domain.
> > 
> > Huh? It would be as simple as Dom0 just figuring out that it is
> > at the end of the available events (b/c its close to the MAX) and
> > then transitioning to the 3-level one (if it can).
> > 
> 
> We need to manipulate various data structures. IMHO this makes
> implementation tricky and racy.
> 
> > > 
> > > For both cases, what do you have in mind for the metric that used to
> > > trigger the switch? How much RAM / how many CPUs is the threshold?
> > 
> > No idea. At what point are we running out of the events?
> 
> In practice, if you have hundreds or thousands of guests running you're
> likely of running out of event channels. I presume host capable of doing
> this has lots of RAM and CPUs...

OK. So "lots" is more than 64GB and 16 CPUS?
> 
> 
> Wei.
> 

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