Subject: Re: High contention on the sk_buff_head.lock
From: Sven-Thorsten Dietrich <sven@thebigcorporation.com>
To: David Miller <davem@davemloft.net>
Cc: ghaskins@novell.com, vernux@us.ibm.com, andi@firstfloor.org,
       netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
       linux-rt-users@vger.kernel.org, pmullaney@novell.com
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Date: Wed, 18 Mar 2009 18:13:11 -0700
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On Wed, 2009-03-18 at 18:03 -0700, David Miller wrote:
> From: Gregory Haskins <ghaskins@novell.com>
> Date: Wed, 18 Mar 2009 17:54:04 -0400
> 
> > Note that -rt doesnt typically context-switch under contention anymore
> > since we introduced adaptive-locks.  Also note that the contention
> > against the lock is still contention, regardless of whether you have -rt
> > or not.  Its just that the slow-path to handle the contended case for
> > -rt is more expensive than mainline.  However, once you have the
> > contention as stated, you have already lost.
> 
> First, contention is not implicitly a bad thing.
> 

Its a bad thing when it does not scale.

> Second, if the -rt kernel is doing adaptive spinning I see no
> reason why that adaptive spinning is not kicking in here to
> make this problem just go away.
> 

If only the first of N contending threads gets to spin, 2..N would
context-switch.


> This lock is held for mere cycles, just to unlink an SKB from
> the networking qdisc, and then it is immediately released.

For very short hold times, and heavy contention, as well as for
scalability, the solution may lie in tunable spinner-count and adaptive
spinner time-out.

Sven

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