netdev - Re: setsockopt()

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Message-Id: <20080709225035.a4d39cb5.billfink@mindspring.com>
Date:	Wed, 9 Jul 2008 22:50:35 -0400
From:	Bill Fink <billfink@...dspring.com>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	Roland Dreier <rdreier@...co.com>,
	David Miller <davem@...emloft.net>, aglo@...i.umich.edu,
	shemminger@...tta.com, netdev@...r.kernel.org, rees@...ch.edu,
	bfields@...ldses.org
Subject: Re: setsockopt()

On Wed, 9 Jul 2008, Evgeniy Polyakov wrote:

> On Wed, Jul 09, 2008 at 11:10:31AM -0700, Roland Dreier (rdreier@...co.com) wrote:
> > so somehow setting the window helps with the scheduling of
> > processes... I guess autotuning lets some queue get too long or
> > something like that.  The actual window doesn't matter too much, since
> > the delay of the network is low enough that even though the bandwidth is
> > very high, the BDP is quite small.  (With a 25 usec RTT, a 128 KB window
> > should be enough for 40 Gbps, well over the raw link speed of 16 Gbps
> > that I have)
> 
> That may be cache issues: depending on what application does it can be
> useful or not to be bound to the same CPU. I suppose if benchmark looks
> into the packet content, then it likely wants to be on the same CPU to
> aliminate cache line ping-pongs, otherwise it only needs to be awakened
> to send/receive next chunk, so having it on different CPU may result in
> better its utilization...

In my own network benchmarking experience, I've generally gotten the
best performance results when the nuttcp application and the NIC
interrupts are on the same CPU, which I understood was because of
cache effects.

I wonder if the "-w128" forces the socket buffer to a small enough size
that it totally fits in cache and this helps the performance.

						-Bill
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