netdev - Re: [PATCH] tcp: refine autocork condition check

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Date:	Wed, 15 Oct 2014 04:47:49 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Weiping Pan <panweiping3@...il.com>
Cc:	netdev@...r.kernel.org, edumazet@...gle.com
Subject: Re: [PATCH] tcp: refine autocork condition check

On Wed, 2014-10-15 at 18:34 +0800, Weiping Pan wrote:
> Inspired by commit b2532eb9abd8 (tcp: fix ooo_okay setting vs Small Queues).
> 
> The last check in tcp_should_autocork() was meant to check that whether we
> only have an ACK in Qdisc/NIC queues, or if TX completion was delayed after we
> processed ACK packet, if so, we should push the packet immediately instead of
> corking it.
> Therefore we should compare sk_wmem_alloc with SKB_TRUESIZE(1) instead of
> skb->truesize.
> 
> After this patch, tcp should have more chances to be corked, and the
> performance should be a little better.  And netperf shows that this patch
> works as expected.
> 
> ./super_netperf.sh 300 -H 10.16.42.249 -t TCP_STREAM -- -m 1 -M 1
> 	      speed	TCPAutoCorking
> Before patch: 169.38    222278
> After patch:  173.27    232988
> 

I do not see how this patch changes anything on this workload, I suspect
noise in your tests ? Full nstat output would give some hints maybe.

TCP_STREAM netperfs send no ACK packets at all.

I am concerned that this patch adds some latencies, and this wont be
seen with your TCP_STREAM test.

Autocorking is a trade off between throughput and latencies.

We need extensive tests, using TCP_RR with various sizes.

Existing behavior is telling that if a prior packet is in qdisc, and
this skb has a bigger truesize, we do not autocork.

In practice, you might hold now packets that are quite big, (more than
SKB_WITH_OVERHEAD(2048 - MAX_TCP_HEADER) bytes of payload.

Typical cases is applications using two writes, one to send a small
header, one for the body of the request/answer.

Existing code is better because we allow the second send() to be pushed
to the qdisc/NIC, before first send is TX completed.

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