netdev - Re: [PATCH v3 net-next 2/2 tuntap: Increase the number of queues in tun.

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Message-ID: <20141203095210.GC9487@redhat.com>
Date:	Wed, 3 Dec 2014 11:52:10 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Pankaj Gupta <pagupta@...hat.com>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	davem@...emloft.net, jasowang@...hat.com, dgibson@...hat.com,
	vfalico@...il.com, edumazet@...gle.com, vyasevic@...hat.com,
	hkchu@...gle.com, wuzhy@...ux.vnet.ibm.com, xemul@...allels.com,
	therbert@...gle.com, bhutchings@...arflare.com, xii@...gle.com,
	stephen@...workplumber.org, jiri@...nulli.us,
	sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH v3 net-next 2/2 tuntap: Increase the number of queues in
 tun.

On Wed, Dec 03, 2014 at 12:49:37PM +0530, Pankaj Gupta wrote:
> Networking under kvm works best if we allocate a per-vCPU RX and TX
> queue in a virtual NIC. This requires a per-vCPU queue on the host side.
> 
> It is now safe to increase the maximum number of queues.
> Preceding patche: 'net: allow large number of rx queues'

s/patche/patch/

> made sure this won't cause failures due to high order memory
> allocations. Increase it to 256: this is the max number of vCPUs
> KVM supports.
> 
> Signed-off-by: Pankaj Gupta <pagupta@...hat.com>
> Reviewed-by: David Gibson <dgibson@...hat.com>

Hmm it's kind of nasty that each tun device is now using x16 memory.
Maybe we should look at using a flex array instead, and removing the
limitation altogether (e.g. make it INT_MAX)?


> ---
>  drivers/net/tun.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index e3fa65a..a19dc5f8 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -113,10 +113,11 @@ struct tap_filter {
>  	unsigned char	addr[FLT_EXACT_COUNT][ETH_ALEN];
>  };
>  
> -/* DEFAULT_MAX_NUM_RSS_QUEUES were chosen to let the rx/tx queues allocated for
> - * the netdevice to be fit in one page. So we can make sure the success of
> - * memory allocation. TODO: increase the limit. */
> -#define MAX_TAP_QUEUES DEFAULT_MAX_NUM_RSS_QUEUES
> +/* MAX_TAP_QUEUES 256 is chosen to allow rx/tx queues to be equal
> + * to max number of vCPUS in guest. Also, we are making sure here
> + * queue memory allocation do not fail.

It's not queue memory allocation anymore, is it?
I would say "
This also helps the tfiles field fit in 4K, so the whole tun
device only needs an order-1 allocation.
"

> + */
> +#define MAX_TAP_QUEUES 256
>  #define MAX_TAP_FLOWS  4096
>  
>  #define TUN_FLOW_EXPIRE (3 * HZ)
> -- 
> 1.8.3.1
> 
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