netdev - Re: Question about __alloc

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Date:	Sat, 04 Dec 2010 15:47:38 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Junchang Wang <junchangwang@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Question about __alloc_skb() speedup

Le samedi 04 décembre 2010 à 22:18 +0800, Junchang Wang a écrit :

> I added the prefetchw() in pktgen as follows:
> 
> diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> index 2953b2a..512f1ae 100644
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -2660,6 +2660,7 @@ static struct sk_buff *fill_packet_ipv4(struct net_device *odev,
>  		sprintf(pkt_dev->result, "No memory");
>  		return NULL;
>  	}
> +	prefetchw(skb->data);
>  
>  	skb_reserve(skb, datalen);
>  
> This time, I can check it without rebooting the system. The performance 
> gain is 4%-5%(stable). Does 4% worth submitting it to the kernel?

Yes I believe so, pktgen being very specific, but I have few questions :

Is it with SLUB or SLAB ?

How many buffers in TX ring on you nic (ethtool -g eth0) ?

What is the datalen value here ? (you prefetch, then advance skb->data)

32 or 64bit kernel ?

How many pps do you get before and after patch ?

Thanks


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