Re: [dpdk-dev] [PATCH] eal: default to one memory channel if not specified

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Oct 14, 2015 at 03:04:43PM +0300, Panu Matilainen wrote:
> On 10/14/2015 02:45 PM, David Marchand wrote:
> >Hello Panu,
> >
> >On Wed, Oct 14, 2015 at 12:22 PM, Panu Matilainen <pmatilai@redhat.com
> ><mailto:pmatilai@redhat.com>> wrote:
> >
> >    Obtaining the correct value, especially from a running system, can
> >    be anything from difficult to plain impossible.  Since the value is
> >    merely an optimization and does not affect functionality otherwise,
> >    its pointless to force such a guess on users initially, such things
> >    belong to performance tuning phase.
> >
> >    Signed-off-by: Panu Matilainen <pmatilai@redhat.com
> >    <mailto:pmatilai@redhat.com>>
> >    ---
> >      lib/librte_eal/common/eal_common_options.c | 10 ++--------
> >      1 file changed, 2 insertions(+), 8 deletions(-)
> >
> >    diff --git a/lib/librte_eal/common/eal_common_options.c
> >    b/lib/librte_eal/common/eal_common_options.c
> >    index 1f459ac..28f10a2 100644
> >    --- a/lib/librte_eal/common/eal_common_options.c
> >    +++ b/lib/librte_eal/common/eal_common_options.c
> >    @@ -104,7 +104,7 @@ eal_reset_internal_config(struct internal_config
> >    *internal_cfg)
> >
> >             internal_cfg->memory = 0;
> >             internal_cfg->force_nrank = 0;
> >    -       internal_cfg->force_nchannel = 0;
> >    +       internal_cfg->force_nchannel = 1;
> >
> >
> >Well, not too sure about this default value.
> >
> >- mempool code is already checking for the 0 value.
> 
> Yeah, I noticed it already handles the zero case.
> 
> >- API already tells for rte_memory_get_nchannel() :
> >  * @return
> >  *   The number of memory channels on the system. The value is 0 if
> >unknown
> >  *   or not the same on all devices.
> 
> ...but missed this one, and thought it'd be "safer" to return some non-zero
> value since callers might be expecting it to be a valid -n value.
> 
> >
> >So, I would let it 0.
> 
> Right, so just drop the default value, reword commit message accordingly and
> resend. Will do unless there are other objections.
> 
> 	- Panu -

I was going to suggest using 4 as the default value, since the channel spreading
should work as designed on systems with either 1, 2 or 4 active channels.

However, given the zero-check inside the mempool code, maybe the default should
be set there instead of in the EAL. [I just don't think the default should be 1.]

Anyone else any other thoughts on this?

/Bruce