netdev - Re: [RFC] net: bump the default number of RSS queues

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Date:   Wed, 4 Apr 2018 17:08:45 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, brouer@...hat.com,
        alexander.duyck@...il.com, oss-drivers@...ronome.com
Subject: Re: [RFC] net: bump the default number of RSS queues

On Tue, 3 Apr 2018 17:20:49 -0700, Eric Dumazet wrote:
> On 04/03/2018 05:14 PM, Jakub Kicinski wrote:
> > Some popular NIC vendors are not adhering to
> > netif_get_num_default_rss_queues() which leads to users being
> > surprised and filing bugs :)  Bump the number of default RX
> > queues to something more reasonable for modern machines.
> > 
> > Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> > ---
> > I'm mostly wondering what's the policy on this default?  When
> > should it be applied?  Why was 8 chosen as the default?  We
> > can abandon using netif_get_num_default_rss_queues() for the
> > nfp but I wonder what's the correct course of action here...
> > Should new drivers use netif_get_num_default_rss_queues() for
> > example?
> > 
> >  include/linux/netdevice.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index 2a2d9cf50aa2..26fe145ada2a 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -3260,7 +3260,7 @@ static inline unsigned int get_netdev_rx_queue_index(
> >  }
> >  #endif
> >  
> > -#define DEFAULT_MAX_NUM_RSS_QUEUES	(8)
> > +#define DEFAULT_MAX_NUM_RSS_QUEUES	(64)
> >  int netif_get_num_default_rss_queues(void);  
> 
> There is no evidence having so many queues is beneficial.
> 
> Too many queues -> lots of overhead in many cases.
> 
> So I would rather not touch this, unless you can present good numbers ;)

Thank you for the comment!  I don't have convincing number it was more
of a matter of consistency :)  

Now I think I forgot about aRFS, when aRFS support for the nfp is added
we will probably start ignoring the default as well.