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

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Date:   Tue, 3 Apr 2018 17:20:49 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>, davem@...emloft.net,
        netdev@...r.kernel.org
Cc:     brouer@...hat.com, alexander.duyck@...il.com,
        oss-drivers@...ronome.com
Subject: Re: [RFC] net: bump the default number of RSS queues



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 ;)