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Message-ID: <1269382380.2915.40.camel@edumazet-laptop>
Date:	Tue, 23 Mar 2010 23:13:00 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	alexander.h.duyck@...el.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH] net: add additional lock to qdisc to increase
 enqueue/dequeue fairness

Le mardi 23 mars 2010 à 14:45 -0700, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Tue, 23 Mar 2010 21:54:27 +0100
> 
> > Quite frankly, the real problem in this case is not the reduced
> > throughput, but fact that one cpu can stay a long time doing the xmits
> > to device, of skb queued by other cpus. This can hurt latencies a lot,
> > for real time threads for example...
> > 
> > I wonder if ticket spinlocks are not the problem. Maybe we want a
> > variant of spinlocks, so that cpu doing transmits can get the lock
> > before other cpus...
> 
> I want to note that things operate the way they do now
> intentionally.
> 
> Herbert Xu and Jamal Hadi Salim were active in this area
> about 4 years ago.

Yes, but ticket spinlocks were added after their work (in 2008 - 2.6.25
if I remember well) and change things.

We want cpu owning __QDISC_STATE_RUNNING being able to re-get the lock
as fast as possible. Alexander results can show the possible speedup.



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