netdev - RE: Is 3.18 patch "The tcp: TSO packets automatic sizing" will cause to Troughput drop when enable NO_HZ_IDEL or HIGH_RES

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Date:	Wed, 7 May 2014 14:33:49 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'V JobNickname' <workofv@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: Is 3.18 patch "The tcp: TSO packets automatic sizing" will
 cause to Troughput drop when enable NO_HZ_IDEL or HIGH_RES_TIMERS ?

From: V JobNickname
> 2014-05-07 0:41 GMT+08:00 Eric Dumazet <eric.dumazet@...il.com>:
> > On Tue, 2014-05-06 at 09:30 -0700, Eric Dumazet wrote:
> >
> >> I cant see any effect for this patch explaining a 50% reduction in
> >> speed, unless a special qdisc setup maybe.
> >
> > Oh wait, a buggy driver could explain the problem.
> >
> > Some drivers really expected TX completion to happen as a side effect of
> > subsequent transmits.
> >
> > You'll have to give more details on the configuration you are using.

> Does the "buggy driver" here mean the NIC driver or some higher level
> network stack driver?
> The xmit function of the used NIC driver is to stuff DMA channel, is
> there have "expected Tx completion" behavior in the driver's flow ?

Some of the TCP transmit flow control requires that the NIC driver
free the skb as soon as it is transmitted (if not before) and also
to limit the number of bytes of data queued for transmit.

This is done to give fair(er) access to the network when there are
some connections doing file transfer with large window and others
doing command-response.

	David