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

On Mon, Apr 23, 2012 at 1:11 PM, Eric Dumazet [off-list ref] wrote:

From: Eric Dumazet <edumazet@google.com>

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