netdev - Re: Latency difference between fifo and pfifo

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Message-ID: <88e4825e-4efe-4163-bb24-299a20aab66d@jasiiieee>
Date:	Wed, 07 Dec 2011 19:34:12 -0500 (EST)
From:	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	David Laight <David.Laight@...LAB.COM>, netdev@...r.kernel.org,
	Rick Jones <rick.jones2@...com>,
	Dave Taht <dave.taht@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: Latency difference between fifo and pfifo_fast



----- Original Message -----
> From: "Stephen Hemminger" <shemminger@...tta.com>
> To: "John A. Sullivan III" <jsullivan@...nsourcedevel.com>
> Cc: "David Laight" <David.Laight@...LAB.COM>, netdev@...r.kernel.org, "Rick Jones" <rick.jones2@...com>, "Dave Taht"
> <dave.taht@...il.com>, "Eric Dumazet" <eric.dumazet@...il.com>
> Sent: Wednesday, December 7, 2011 6:27:09 PM
> Subject: Re: Latency difference between fifo and pfifo_fast
> 
> 
> > <grin> Sorry to have kicked up a storm! We really don't have a
> > problem - just trying to optimize our environment.  We have been
> > told by our SAN vendor that, because of the 4KB limit on block
> > size in Linux file systems, iSCSI connections for Linux file
> > services are latency bound and not bandwidth bound.  I'm not sure
> > if I believe that based upon our traces where tag queueing seems
> > to coalesce SCSI commands into larger blocks and we are able to
> > achieve network saturation.  I was just wondering, since it is all
> > the same traffic and hence no need to separate into bands, if I
> > should change the qdisc on those connections from pfifo_fast
> > (which I assume needs to look at the TOS bits, sort into bands,
> > and poll the separate bands) to fifo which I assume simply dumps
> > packets on the wire.  Thanks - John
> 
> Is this a shared network? TOS won't matter if it is only your
> traffic.
> 
> There are number of route metrics that you can tweak to that can
> reduce TCP slow
> start effects, like increasing the initial cwnd, etc.
> 
It is a private network dedicated only to SAN traffic - a couple of SAN devices and some virtualization hosts - John
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