netdev - Re: [RFC] network driver skb allocations

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Message-ID: <1272916166.27948.62.camel@localhost>
Date:	Mon, 03 May 2010 20:49:26 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] network driver skb allocations

On Mon, 2010-05-03 at 19:06 +0200, Eric Dumazet wrote:
[...]
> Current logic for drivers is to :
> 
> allocate skbs (sk_buff + data) and put them in a ring buffer.

Not all of them.

> When rx interrupt comes, get the skb and give it to stack.
> 
> Allocate a new skb (sk_buff + data) and put it in rx fat ring buffer (511 entries for tg3 )
> 
> This is suboptimal, because sk_buff will probably be cold 512 rx later...
> Also, NUMA info might be wrong : sk_buff should be allocated on current node,
> not on the device preferred node.

This also avoids allocating sk_buffs that are never needed due to GRO or
scattering of jumbo frames.

> Drivers should allocate only the data part for NIC, and at the time of interrupt,
> allocate the skb_buff and link it to buffer filled by NIC.

I think we found that this increases latency, so sfc switches between
page and skb allocations dynamically.

Ben.

> With a prefetch(first_cache_line_of_data) before doing sk_buff allocation and init,
> eth_type_trans() / get_rps_cpus() would be much faster.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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