Subject: Re: [PATCH v4 net-next 2/2 tuntap: Increase the number of queues in tun.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 6 Jan 2015 11:49:12 +0200

On Tue, Jan 06, 2015 at 11:09:16AM +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 patch: 'net: allow large number of rx queues'
> 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.
> 
> Size of tun_struct changes from 8512 to 10496 after this patch. This keeps 
> pages allocated for tun_struct before and after the patch to 3. 
> 
> Signed-off-by: Pankaj Gupta <[email-masked]>
> Reviewed-by: David Gibson <[email-masked]>
> ---
>  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.

VCPUs I think.

> Also, we are making sure here
> + * queue memory allocation do not fail.

What does this mean? How are we making sure?
I would drop this phrase really.

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