netdev - Re: [PATCH] tcp: splice as many packets as possible at once

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Message-Id: <20090113.153104.72134741.davem@davemloft.net>
Date:	Tue, 13 Jan 2009 15:31:04 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	ben@...s.com, w@....eu, jarkao2@...il.com, mingo@...e.hu,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: [PATCH] tcp: splice as many packets as possible at once

From: Eric Dumazet <dada1@...mosbay.com>
Date: Fri, 09 Jan 2009 16:42:44 +0100

> David, if you referred to code at line 1374 of net/ipv4/tcp.c, I
> believe there is no issue with it. We really want to break from this
> loop if !timeo .

Correct, I agree, and I gave some detailed analysis of this in
another response :-)

> Willy patch makes splice() behaving like tcp_recvmsg(), but we might call
> tcp_cleanup_rbuf() several times, with copied=1460 (for each frame processed)

"Like", sure, but not the same since splice() lacks the low-water
and backlog checks.

> I wonder if the right fix should be done in tcp_read_sock() : this is the
> one who should eat several skbs IMHO, if we want optimal ACK generation.
> 
> We break out of its loop at line 1246
> 
> if (!desc->count) /* this test is always true */
> 	break;
> 
> (__tcp_splice_read() set count to 0, right before calling tcp_read_sock())
> 
> So code at line 1246 (tcp_read_sock()) seems wrong, or pessimistic at least.

Yes, that's very odd.
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