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

From: Ingo Molnar <[email-masked]>
Date: Thu, 30 Nov 2006 07:47:58 +0100

* David Miller <[email-masked]> 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