netdev - Re: [PATCH] ifb: add multi-queue support

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Message-ID: <4AFCF50D.6040106@gmail.com>
Date:	Fri, 13 Nov 2009 06:56:29 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <shemminger@...tta.com>,
	Patrick McHardy <kaber@...sh.net>,
	Tom Herbert <therbert@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] ifb: add multi-queue support

Changli Gao a écrit :
> 2009/11/12 Eric Dumazet <eric.dumazet@...il.com>:
>> I believe this patch is fine, but maybe Jarek concern about workqueue
>> vs tasklet should be addressed...
>>
>> We could use the previous handling in case numtxqs==1 , ie use a tasklet
>> instead of a work queue ?
> 
> I don't think it is a good idea. If we do so, the code will be messy,
> and lost the flexibility of process. In fact, latency isn't a problem
> when system load isn't high, and when system load is high (due to too
> many NIC IRQs), throughput and interaction is more important, and the
> current linux networking subsystem just dose so through the softirqd
> mechanism.
> 

Messy ? Because of few tests added in code, and branches always
correctly predicted ?

Still some people might rely on tasklet instead of workqueues
and added scheduler stress and latency penalty. Tasklet are softirq
and normally are processed a few nanosecs later than RX softirq, 
on the same CPU, while with your workqueue, I guess the scheduler will
try to not migrate it, so we add a penalty for light to moderate load.

I guess this new ifb mode would be a regression for them ?

If you dont want to maintain a compatibility mode, maybe you
should introduce a complete new driver, drivers/net/ifbmq.c or ifbwq.c

(multiqueue or workqueue references)
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