netdev - Re: Kernel rwlock design, Multicore and IGMP

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Message-ID: <1289969017.2732.82.camel@edumazet-laptop>
Date:	Wed, 17 Nov 2010 05:43:37 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cypher Wu <cypher.w@...il.com>
Cc:	Chris Metcalf <cmetcalf@...era.com>,
	Américo Wang <xiyou.wangcong@...il.com>,
	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: Kernel rwlock design, Multicore and IGMP

Le mercredi 17 novembre 2010 à 09:30 +0800, Cypher Wu a écrit :
> O
> struct in_device::mr_ifc_timer. Every time when a process join/leave a
> MC group, igmp_ifc_event() -> igmp_ifc_start_timer() will start the
> timer on the core that the system call issued, and
> igmp_ifc_timer_expire() will be called on that core in the bottem halt
> of timer interrupt.
> If we call join/leave on mutlicores that timers will run on all these
> cores, but it seems only one or two will generate IGMP message, others
> will only lock the list and loop throught it with nothing generated.
> 
> 

Problem would not be timer being restarted on different cores (very
small impact), but scanning a list in igmpv3_send_cr() with many items
in it and expensive things, under timer handler (softirq), so adding
spikes of latency.

IGMP_Unsolicited_Report_Interval is 10 seconds, so we start timer in a 5
second average.

I am not sure there is a need to join/leave thousand of groups per
second anyway...



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