netdev - Re: Latency difference between fifo and pfifo

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Message-ID: <1323263079.2312.22.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Wed, 07 Dec 2011 14:04:39 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>
Cc:	Dave Taht <dave.taht@...il.com>, netdev@...r.kernel.org,
	Rick Jones <rick.jones2@...com>
Subject: Re: Latency difference between fifo and pfifo_fast

Le mardi 06 décembre 2011 à 14:44 -0500, John A. Sullivan III a écrit :
> Interesting.  Would that still be true if all the traffic is the same,
> i.e., nothing but iSCSI packets on the network? Or would just dumping
> packets with minimal processing be fastest? Thanks - John

Dave focuses on fairness and latencies under ~20 ms (a typical (under)
provisioned ADSL (up)link shared by many (hostile) flows, with various
type of services)

I doubt this is your concern ? You want high throughput more than low
latencies ...

Your workload is probably under _one_ ms latencies, and dedicated link
to address few targets.

If you have to use a Qdisc (and expensive packet classification), then
something is wrong in your iSCSI network connectivity :)

Please note that with BQL, the NIC TX ring size doesn’t matter, and you
could get "Virtual device ethX asks to queue packet!" warnings in your
message log.

So before removing Qdisc, you also want to make sure BQL is disabled for
your NIC device/queues.
(BQL is scheduled for linux-3.3)



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