Subject: Re: [PATCH] net: skb_tx_hash() improvements

From: Eric Dumazet <[email-masked]>
Date: Mon, 04 May 2009 08:12:31 +0200

David Miller a écrit :
> From: David Miller <[email-masked]>
> Date: Fri, 01 May 2009 09:17:47 -0700 (PDT)
> 
>> From: Eric Dumazet <[email-masked]>
>> Date: Fri, 01 May 2009 11:29:54 +0200
>>
>>> -	} else if (skb->sk && skb->sk->sk_hash) {
>>> +		/*
>>> +		 * Try to avoid an expensive divide, for symmetric setups :
>>> +		 *   number of tx queues of output device ==
>>> +		 *   number of rx queues of incoming device
>>> +		 */
>>> +		if (hash >= dev->real_num_tx_queues)
>>> +			hash %= dev->real_num_tx_queues;
>>> +		return hash;
>>> +	}
>> Subtraction in a while() loop is almost certainly a lot
>> faster.
> 
> To move forward on this, I've commited the following to
> net-next-2.6, thanks!
> 
> net: Avoid modulus in skb_tx_hash() for forwarding case.
> 
> Based almost entirely upon a patch by Eric Dumazet.
> 
> The common case is to have num-tx-queues <= num_rx_queues
> and even if num_tx_queues is larger it will not be significantly
> larger.
> 
> Therefore, a subtraction loop is always going to be faster than
> modulus.
> 
> Signed-off-by: David S. Miller <[email-masked]>
> ---
>  net/core/dev.c |    8 ++++++--
>  1 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 8144295..3c8073f 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -1735,8 +1735,12 @@ u16 skb_tx_hash(const struct net_device *dev, const struct sk_buff *skb)
>  {
>  	u32 hash;
>  
> -	if (skb_rx_queue_recorded(skb))
> -		return skb_get_rx_queue(skb) % dev->real_num_tx_queues;
> +	if (skb_rx_queue_recorded(skb)) {
> +		hash = skb_get_rx_queue(skb);
> +		while (unlikely (hash >= dev->real_num_tx_queues))
> +			hash -= dev->real_num_tx_queues;
> +		return hash;
> +	}
>  
>  	if (skb->sk && skb->sk->sk_hash)
>  		hash = skb->sk->sk_hash;

Yes, I checked that compiler did not use a divide instruction here
(I remember it did on a similar loop in kernel, related to time)

Thank you