netdev - Re: [PATCH v6] net: batch skb dequeueing from softnet input_pkt

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Message-ID: <20100503070925.572bbee6@infradead.org>
Date:	Mon, 3 May 2010 07:09:25 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>, hadi@...erus.ca,
	xiaosuo@...il.com, therbert@...gle.com, shemminger@...tta.com,
	netdev@...r.kernel.org, lenb@...nel.org
Subject: Re: [PATCH v6] net: batch skb dequeueing from softnet
 input_pkt_queue

On Mon, 3 May 2010 12:34:26 +0200
Andi Kleen <andi@...stfloor.org> wrote:

> > > Maybe its low cost, (apparently, it is, since I can reach ~900.000
> > > ipis on my 16 cores machine) but multiply this by 16 or 32 or 64
> > > cpus, and clockevents_notify() cost appears to be a killer, all
> > > cpus compete on a single lock.
> > > 
> > > Maybe this notifier could use RCU ?
> > 
> > could this be an artifact of the local apic stopping in deeper C
> > states? (which is finally fixed in the Westmere generation)
> 
> Yes it is I think.
> 
> But I suspect Eric wants a solution for Nehalem.

sure ;-)


so the hard problem is that on going idle, the local timers need to be
funneled to the external HPET. Afaik right now we use one channel of
the hpet, with the result that we have one global lock for this.

HPETs have more than one channel (2 or 3 historically, newer chipsets
iirc have a few more), so in principle we can split this lock at least
a little bit... if we can get to one hpet channel per level 3 cache
domain we'd already make huge progress in terms of cost of the
contention....



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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