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

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Date:	Thu, 30 Nov 2006 07:17:58 +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:

> We can make explicitl preemption checks in the main loop of 
> tcp_recvmsg(), and release the socket and run the backlog if 
> need_resched() is TRUE.
> 
> This is the simplest and most elegant solution to this problem.

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).

[ Also note that "sprinkle the code with preempt_disable()" kind of
  solutions, besides hurting interactivity, are also a pain to resolve 
  in something like PREEMPT_RT. (unlike say a spinlock, 
  preempt_disable() is quite opaque in what data structure it protects, 
  etc., making it hard to convert it to a preemptible primitive) ]

> The one suggested in your patch and paper are way overkill, there is 
> no reason to solve a TCP specific problem inside of the generic 
> scheduler.

agreed.

What we could also add is a /reverse/ mechanism to the scheduler: a task 
could query whether it has just a small amount of time left in its 
timeslice, and could in that case voluntarily drop its current lock and 
yield, and thus give up its current timeslice and wait for a new, full 
timeslice, instead of being forcibly preempted due to lack of timeslices 
with a possibly critical lock still held.

But the suggested solution here, to "prolong the running of this task 
just a little bit longer" only starts a perpetual arms race between 
users of such a facility and other kernel subsystems. (besides not being 
adequate anyway, there can always be /so/ long lock-hold times that the 
scheduler would have no other option but to preempt the task)

	Ingo
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