netdev - Re: Latency difference between fifo and pfifo

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Date:	Tue, 06 Dec 2011 03:39:56 -0500 (EST)
From:	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Latency difference between fifo and pfifo_fast



----- Original Message -----
> From: "Eric Dumazet" <eric.dumazet@...il.com>
> To: "John A. Sullivan III" <jsullivan@...nsourcedevel.com>
> Cc: netdev@...r.kernel.org
> Sent: Tuesday, December 6, 2011 1:29:02 AM
> Subject: Re: Latency difference between fifo and pfifo_fast
> 
> Le mardi 06 décembre 2011 à 07:02 +0100, Eric Dumazet a écrit :
> > Le lundi 05 décembre 2011 à 23:10 -0500, John A. Sullivan III a
> > écrit :
> > > Hello, all.  We are trying to minimize latency on our iSCSI SAN.
> > >  The
> > > network is entirely dedicated to the iSCSI traffic.  Since all
> > > the
> > > traffic is the same, would it make sense to change the qdisc for
> > > that
> > > interface to fifo from the default pfifo_fast or is the latency
> > > difference between the two completely negligible? Thanks - John
> > 
> > A very small difference indeed. How many packets per second are
> > expected ? What kind of NIC are you using ?
> > 
> 
> To really remove a possible source of latency, you could remove qdisc
> layer...
> 
> ifconfig eth2 txqueuelen 0
> tc qdisc add dev eth2 root pfifo
> tc qdisc del dev eth2 root
> 
> 
> 
> 
Really? I didn't know one could do that.  Thanks.  However, with no queue length, do I have a significant risk of dropping packets? To answer your other response's question, these are Intel quad port e1000 cards.  We are frequently pushing them to near line speed so 1,000,000,000 / 1534 / 8 = 81,486 pps - John
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