netdev - Re: [PATCH net-next] tcp: introduce tcp_try

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Message-Id: <20120423.224602.594997774992725103.davem@davemloft.net>
Date:	Mon, 23 Apr 2012 22:46:02 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ncardwell@...gle.com
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org,
	therbert@...gle.com, maze@...gle.com, ilpo.jarvinen@...sinki.fi
Subject: Re: [PATCH net-next] tcp: introduce tcp_try_coalesce

From: Neal Cardwell <ncardwell@...gle.com>
Date: Mon, 23 Apr 2012 22:39:10 -0400

> On Mon, Apr 23, 2012 at 1:11 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> From: Eric Dumazet <edumazet@...gle.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.
> 
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> 
> Thanks for the background info, Eric.

Applied, thanks Eric.

Although I'd like to ask you to clean up tcp_try_coalesce() a bit.

It effectively returns a boolean, but you've clouded this up by
returning an int and defining it in the comment to return "> 0" or
not.

Just make it return a real bool.

I know why you did this, it makes the "eaten" code somewhat simpler in
tcp_data_queue(), but overall it's more confusing how it is now.

People look at how the tcp_try_coalesce() return value is interpreted
and say "in what cases can it return a negative value?"  We both know
it can't, but you have to read the entire function to figure that out.

And that's by definition not intuitive.

Thanks.
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