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Date: Wed, 18 Nov 2020 14:54:40 +0000
From: Bruce Richardson <bruce.richardson@intel.com>
To: Thomas Monjalon <thomas@monjalon.net>
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Subject: Re: [dpdk-dev] [PATCH v12 09/14] build: optional NUMA and cpu
counts detection
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On Wed, Nov 18, 2020 at 03:42:36PM +0100, Thomas Monjalon wrote:
> 18/11/2020 15:19, Juraj Linkeš:
> > From: Thomas Monjalon <thomas@monjalon.net>
> > > 16/11/2020 10:13, Bruce Richardson:
> > > > On Mon, Nov 16, 2020 at 08:24:48AM +0100, Thomas Monjalon wrote:
> > > > > 13/11/2020 15:31, Juraj Linkeš:
> > > > > > +option('max_lcores', type: 'integer', value: 0,
> > > > > > + description: 'maximum number of cores/threads supported by EAL.
> > > > > > +Set to positive integer to overwrite per-arch or cross-compilation
> > > defaults. Set to -1 to detect the number of cores on the build machine.')
> > > option('max_numa_nodes', type: 'integer', value: 0,
> > > > > > + description: 'maximum number of NUMA nodes supported by EAL. Set
> > > > > > +to positive integer to overwrite per-arch or cross-compilation
> > > > > > +defaults. Set to -1 to detect the number of numa nodes on the
> > > > > > +build machine.')
> > > > >
> > > > > First comment: I don't like having so long description.
> > > > > Second: I don't understand.
> > > > >
> > > > > It is said the default value is 0 so I expect it means automatic detection.
> > > > > But later it is said -1 is for detection. So ?
> > > > >
> > > > Zero is for the "per-arch or cross-compilation default". This was
> > > > discussed quite a bit in previous versions and this was te best
> > > > compromise we could come up with. Having a default of auto-detect is
> > > > definitely not something I think we should go with - just thinking of
> > > > all the build CI jobs running on
> > > > 2 or 4 core VMs! However, Juraj really felt there was value in having
> > > > auto-detection, so it's set as a -1 value, which I'm ok with.
> > >
> > > The problem is that I don't understand what 0 means.
> > >
> >
> > There are three pieces of information which we need to convey:
> > 1. The default value (0) indicates that per-arch or cross-compilation defaults will be used.
> > 2. Positive integer values will be used instead of these defaults.
>
> Where these positive values come from?
>
> > 3. Detected values will be used for native build when the value is -1.
>
> Why not detect for any native build set up with 0 (default)?
>
That was one of the original suggestions, but I strongly disagreed with
that, because many builds are done on hardware very different from the
final deployment. It would mean that any builds done in e.g. jenkins or
travis, with a 2-core vm, would be limited to running with two cores only.