Re: [Xen-devel] schedulers and topology exposing questions

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Wed, 2016-01-27 at 10:27 -0500, Konrad Rzeszutek Wilk wrote:
> On Wed, Jan 27, 2016 at 03:10:01PM +0000, George Dunlap wrote:
> >Â
> > I'm not sure I understand the situation right, but it sounds a bit
> > like
> > what you're seeing is just a quirk of the fact that Linux doesn't
> > always
> > send IPIs to wake other processes up (either by design or by
> > accident),
> 
> It does and it does not :-)
> 
> > but relies on scheduling timers to check for work to
> > do.ÂÂPresumably
> 
> It .. I am not explaining it well. The Linux kernel scheduler when
> called for 'schedule' (from the UDP sendmsg) would either pick the
> next
> appliction and do a context swap - of if there were none - go to
> sleep.
> [Kind of - it also may do an IPI to the other CPU if requested ,but
> that requires
> some hints from underlaying layers]
> Since there were only two apps on the runqueue - udp sender and udp
> receiver
> it would run them back-to back (this is on baremetal)
> 
> However if SMT was not exposed - the Linux kernel scheduler would put
> those
> on each CPU runqueue. Meaning each CPU only had one app on its
> runqueue.
> 
> Hence no need to do an context switch.
> [unless you modified the UDP message to have a timeout, then it would
> send an IPI]
>
So, may I ask what piece of (Linux) code are we actually talking about?
Because I had a quick look, and could not find where what you describe
happens....

Thanks and Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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