linux-kernel - Re: [tbench regression fixes]: digging out smelly deadmen.

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Message-Id: <20081031.165144.86556444.davem@davemloft.net>
Date:	Fri, 31 Oct 2008 16:51:44 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	zbr@...emap.net, shemminger@...tta.com, ilpo.jarvinen@...sinki.fi,
	rjw@...k.pl, mingo@...e.hu, s0mbre@...rvice.net.ru,
	a.p.zijlstra@...llo.nl, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, efault@....de, akpm@...ux-foundation.org
Subject: Re: [tbench regression fixes]: digging out smelly deadmen.

From: Eric Dumazet <dada1@...mosbay.com>
Date: Fri, 31 Oct 2008 22:03:00 +0100

> Evgeniy Polyakov a écrit :
> > On Fri, Oct 31, 2008 at 12:57:13PM -0700, Stephen Hemminger (shemminger@...tta.com) wrote:
> >> Why bother with last_rx at all on loopback.  I have been thinking
> >> we should figure out a way to get rid of last_rx all together. It only
> >> seems to be used by bonding, and the bonding driver could do the calculation
> >> in its receive handling.
> > Not related to the regression: bug will be just papered out by this
> > changes. Having bonding on loopback is somewhat strange idea, but still
> > this kind of changes is an attempt to make a good play in the bad game:
> > this loopback-only optimization does not fix the problem.
> 
> Just to be clear, this change was not meant to be committed.
> It already was rejected by David some years ago (2005, and 2006)
> 
> http://www.mail-archive.com/netdev@vger.kernel.org/msg07382.html

However, I do like Stephen's suggestion that maybe we can get rid of
this ->last_rx thing by encapsulating the logic completely in the
bonding driver.

> If you read my mail, I was *only* saying that tbench results can be sensible to
> cache line ping pongs. tbench is a crazy benchmark, and only is a crazy benchmark.
> 
> Optimizing linux for tbench sake would be .... crazy ?

Unlike dbench I think tbench is worth cranking up as much as possible.

It doesn't have a huge memory working set, it just writes mostly small
messages over a TCP socket back and forth, and does a lot of blocking

And I think we'd like all of those operating to run as fast as possible.

When Tridge first wrote tbench I would see the expected things at the
top of the profiles.  Things like tcp_ack(), copy to/from user, and
perhaps SLAB.

Things have changed considerably.

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