netdev - Re: [patch 1/4] - Potential performance bottleneck for Linxu TCP

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Message-ID: <20061130064758.GD2003@elte.hu>
Date:	Thu, 30 Nov 2006 07:47:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	wenji@...l.gov, akpm@...l.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/4] - Potential performance bottleneck for Linxu TCP

* David Miller <davem@...emloft.net> wrote:

> > yeah, i like this one. If the problem is "too long locked section", 
> > then the most natural solution is to "break up the lock", not to 
> > "boost the priority of the lock-holding task" (which is what the 
> > proposed patch does).
> 
> Ingo you're mis-read the problem :-)

yeah, the problem isnt too long locked section but "too much time spent 
holding a lock" and hence opening up ourselves to possible negative 
side-effects of the scheduler's fairness algorithm when it forces a 
preemption of that process context with that lock held (and forcing all 
subsequent packets to be backlogged).

but please read my last mail - i think i'm slowly starting to wake up 
;-) I dont think there is any real problem: a tweak to the scheduler 
that in essence gives TCP-using tasks a preference changes the balance 
of workloads. Such an explicit tweak is possible already.

furthermore, the tweak allows the shifting of processing from a 
prioritized process context into a highest-priority softirq context. 
(it's not proven that there is any significant /net win/ of performance: 
all that was proven is that if we shift TCP processing from process 
context into softirq context then TCP throughput of that otherwise 
penalized process context increases.)

	Ingo
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