1:43 a.m.
On Wed, Dec 19, 2012 at 09:53:15AM -0500, Doron Fediuck wrote:
----- Original Message -----
> From: "Dan Kenigsberg" <danken(a)redhat.com>
> To: "Greg Padgett" <gpadgett(a)redhat.com>
> Cc: "engine-devel" <engine-devel(a)ovirt.org>,
vdsm-devel(a)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.
Dan.
We need to be
able to tell the difference. So this moves us to cpuCores2 suggestion.
This is one possibility (cpuRealCores?), and the alternative is an
indication of vdsm config (true/false) which may be removed in the future.
I suspect over time cpu and cpu2 will confuse people.
So I'd suggest having the boolean and removing it along with the vdsm
configuration in the next ovirt version.