Re: [PATCH net-next] tcp: introduce tcp_try_coalesce

[Neal Cardwell <ncardwell@xxxxxxxxxx>]

On Mon, Apr 23, 2012 at 1:11 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
>
> commit c8628155ece3 (tcp: reduce out_of_order memory use) took care of
> coalescing tcp segments provided by legacy devices (linear skbs)
>
> We extend this idea to fragged skbs, as their truesize can be heavy.
>
> ixgbe for example uses 256+1024+PAGE_SIZE/2 = 3328 bytes per segment.
>
> Use this coalescing strategy for receive queue too.
>
> This contributes to reduce number of tcp collapses, at minimal cost, and
> reduces memory overhead and packets drops.

The mechanics look solid, but I'm a little concerned about the
potential added overhead for the new case where tcp_try_coalesce()
does a skb_copy_bits() for in-order data that it is coalescing at the
end of the sk_receive_queue. Do you have any performance numbers for
this case to help suggest whether this added copy is a concern?

neal
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