On Thu, Dec 20, 2012 at 10:23:55AM +0200, Itamar Heim wrote:
> On 12/20/2012 09:43 AM, Dan Kenigsberg wrote:
> >On Wed, Dec 19, 2012 at 09:53:15AM -0500, Doron Fediuck wrote:
> >>
> >>----- Original Message -----
> >>>From: "Dan Kenigsberg" <dan...@redhat.com>
> >>>To: "Greg Padgett" <gpadg...@redhat.com>
> >>>Cc: "engine-devel" <engine-de...@ovirt.org>, vdsm-de...@fedorahosted.org
> >>>Sent: Wednesday, December 19, 2012 3:59:11 PM
> >>>Subject: Re: [Engine-devel] CPU Overcommit Feature
> >>>
> >>>On Mon, Dec 17, 2012 at 09:37:57AM -0500, Greg Padgett wrote:
> >>>>Hi,
> >>>>
> >>>>I've been working on a feature to allow CPU Overcommitment of hosts
> >>>>in a cluster.  This first stage allows the engine to consider host
> >>>>cpu threads as cores for the purposes of VM resource allocation.
> >>>>
> >>>>This wiki page has further details, your comments are welcome!
> >>>>http://www.ovirt.org/Features/cpu_overcommit
> >>>
> >>>I've commented about the vdsm/engine API on
> >>>http://gerrit.ovirt.org/#/c/10144/ but it is probably better to
> >>>reiterate it here.
> >>>
> >>>The suggested API is tightly coupled with an ugly hack we pushed to
> >>>vdsm
> >>>in order not to solve the issue properly on the first strike.
> >>>
> >>>If we had not have report_host_threads_as_cores, I think we'd have a
> >>>simpler API reporting only cpuThreads and cpuCores; with no funny
> >>>boolean flags.
> >>>
> >>>Let us strive to that position as much as we can.
> >>>
> >>>How about asking whoever used report_host_threads_as_cores to unset
> >>>it
> >>>once they install Engine 3.2 ? I think that these are very few
> >>>people,
> >>>that would not mind this very much.
> >>>
> >>>If this is impossible, I'd add a cpuCores2, always reporting the true
> >>>number, to be used by new Engines. We may even report it only on the
> >>>very few cases of report_host_threads_as_cores being set.
> >>>
> >>>Dan.
> >>
> >>Hi Dan,
> >>Thanks for the review.
> >>
> >>I agree simply reporting cores and threads would be the right solution.
> >>However, when you have hyperthreading turned off you get cores=threads.
> >>This is the same situation you have when hyperthreading turned on, and
> >>someone used the vdsm configuration of reporting threads as cores.
> >>
> >>So the engine won't know the real status of the host.
> >
> >This is not surprising, as report_host_threads_as_cores means in blunt
> >English "lie to Engine about the number of cores". The newly suggested
> >flag says "don't believe what I said in cpuCores, since I'm lying". Next
> >thing we'd have is another flag saying that the former flag was a lie,
> >and cpuCores is actually trustworthy.
> >
> >Instead of dancing this dance, I suggest to stop lying.
> >
> >report_host_threads_as_cores was a hack to assist a older Engine
> >versions. Engine users that needed it had to set it out-of-band on their
> >hosts. Now if they upgrade their Engine, they can -- as easily -- reset
> >that value.
> >
> >If they forget, nothing devastating happens beyond Engine assuming that
> >hyperthreading is off.
> >
> >Please consider this suggestion. I find it the simplest for all involved
> >parties.
> the only problem is the new vdsm doesn't know which engine may be using it.
> if engine would say "getVdsCaps engineVersion=3.2", then vdsm could
> know engine no longer needs lying to and ignore the flag, re-using
> same field.

Note that I do not suggest to drop report_host_threads_as_cores now.
I am suggesting to keep on lying even to new Engine.
If someone thinks that lying is bad, he should reset

It seems to me that the suggested API is being coerced by a very limited
use case, that is not going to be really harmed by a straight-forward API.

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